Let's start by ditching some conventional wisdom. Many people seem to be of the opinion that a homeschoolers should take as many outside classes as possible. Though there is nothing wrong with this and it may be the best use of available resources, my experiences have taught me another approach.
Also, contrary to what some think, mothers can successfully guide their children (including sons) through high school.
What Colleges Want
Most top colleges have minimum high school academic requirements. Carefully check out the requirements of all your potential college choices. They generally want four years of math through calculus, four years of English, two to four years of a foreign language, four years of social studies, and four years of science to be competitive. They prefer that applicants take the most rigorous classes available.
Even if the student isn't planning to go to Yale, or to any college at all, that is no excuse to slack off in high school. You are shortchanging your kids if they leave home without a rigorous, substantial knowledge base including science, math, English, and history. They also need lots of practice in thinking and communicating well.
If your daughter just wants to get married and be a homeschool mom, great! Push her to get as much as she can from your homeschool, so she can teach her own someday. Since a homeschool mother will be the teacher and mentor of her kids she needs deep spiritual and intellectual resources to draw on. I would be nothing if it weren't for my mom and her prodigious abilities, which she honed through college and graduate school.
Your Four Options
You have four options for completing course work. The classic homeschool way is self study, in which students learn subjects on their own or from parents. I even designed my own classes when I had a unique need. The other three options- local colleges, homeschool co-ops, and distance learning-differ from each other in the details, but resemble institutional education (i.e. public or private school) methods in having an outside instructor. We term these "institutional methods." A fifth option, apprenticeship, while great for learning real-world skills, is generally not suitable for academic subjects, so we will not discuss it here.
Cost-Benefit Analysis
Any choice has costs and benefits. Academic decisions are no different. Instead of just picking what looks good, try applying rigorous cost-benefit analysis. Costs are fairly straightforward and can be evaluated in terms of monetary expense and time (time commitment and how much will it reduce flexibility). Benefits are more difficult, being either direct (i.e. quality of learning) or indirect (recommendations, help with college positioning efforts, etc.). We compare each option in terms of these parameters.
Concerning Dough
Education is expensive. Self-study usually wins the expense comparison hands down since there is no tuition cost. Institutional methods can vary widely in cost, but co-ops are generally the cheapest, followed by distance learning and local colleges.
Ideally expense wouldn't be an issue, but for many homeschool families it is and they must make their decisions accordingly.
Flexibility
For me, flexibility is one of the most important considerations. One of the greatest benefits of homeschooling is not being chained to a schedule. In my experience many opportunities only present themselves to those who have enough freedom to pursue them.
If you're taking a full load online or at a college, chances are you will be quite limited. While the trade-off may be worth it, remember that inflexibility can limit your opportunities for growth in other areas and prevent you from doing things that might make you more competitive in the college admissions process.
Quality of Learning
Several things will affect how much a given student gets out of a particular class. The student's learning style needs to be accommodated by the style of instruction. For instance, I'm very independent and learn by reading and solving problems on my own schedule. As such, self-study is generally my ideal option, with text-based, asynchronous (meaning that there are no scheduled online meetings) distance learning coming in second. For students who are auditory learners, a more traditional classroom setting might look more attractive.
Before signing up for a class, research it. Does the teacher know the subject well? How much and what kind of interaction will you have with the teacher? An accessible instructor can make a class much more valuable than if the instructor is a talking head who hands out exams. Don't make any assumptions about the nature of a class.
Tangent Skills
Some classes teach you more just the declared subject matter. I took an excellent online biology course a few years ago with the intention of learning biology. However, in addition to knowledge of biology, I came away with greatly improved test taking and study skills, as well as the ability to write quick, test style essays. These "tangent skills" are side benefits whose usefulness transcends the course's subject matter. Whenever you are considering a particular option, especially if it is institutional, do a bit of investigation to see what kinds of tangent skills it might develop.
Validation
To do well in college admissions, you need some amount of outside validation. Validation generally comes in the form of standardized tests or class grades. For homeschoolers, standardized test scores are very important and grades (especially mom-assigned ones) are not weighted very heavily. An advantage of the institutional methods over self-study is that they give official grades, which have somewhat more credibility with admissions officers. However, due to grade inflation, top colleges don't generally assign much value to any grading scheme. The best thing to validate a course is take a standardized test, either AP, IB, or SAT Subject tests. Not all classes will prepare you well for a test, so research this as well.
Self-assigned grades for a class need to accurately reflect any associated standardized test scores. Don't put A+ for the subject on the transcript if the standardized test score for that subject is in the fortieth percentile. This will be a glaring red flag to admissions officers that you are not trustworthy. Get reasonable validation and be honest.
Recommendation Letters
Getting a letter of recommendation is both the most overvalued benefit of taking outside classes and the most frequently botched part of the application process. Recommendations are important, but you seldom need more than two and the recommender needs to be picked with great care. In most cases, a teacher from whom you have taken only one class is probably not the best recommender. Unless you know that a particular teacher will be an exceptional recommender and unless you will have the opportunity to really get to know them well, don't take the class just to get a recommendation.
Academic Curveballs
The ideas presented here can be useful to all kinds of students, not just those interested in top colleges. One thing parents need to know is that academically laid back students can suddenly blossom in high school and turn out to have great passion and aptitude for things academic. Even those who never showed interest in college at all can suddenly change their mind. If you haven't chosen a rigorous preparation program with the appropriate tests, it's easy to sink possibilities for college admission and deny the student an opportunity to fulfill his potential. By now, God has probably thrown you quite a few surprises through your kids. Be prepared, because the next one just might involve a bunch of college applications.
