I think this depends on a lot of different factors. I went with a 2-year, DIY-ish program (I lived in the Boston area already, so Harvard Extension was a really easy choice for me). Here are some of my thoughts:
On one hand, if you can shorten your timeline by a year, that's an extra year of attending salary down the line, which means paying an extra $24k is easily worth it financially. On the other hand, maybe you could have put together a 1-year program on your own anyways. On the other other hand, if an extra year helps you strengthen your application so much that you can get significant scholarship money, that counts for a lot too.
I decided to go with a 2-year program largely because it gave me more time to explore medicine, do shadowing, get clinical exposure, volunteer, do research, etc. A 1-year formal post-bac follows a pretty intense schedule and may not leave a ton of time to do lots of EC's. I also felt that taking an extra year would make me look less capricious: there is less of the, "I woke up today and decided I was going to go to med school" feeling to my app. I didn't really have a whole lot of medical EC's prior to starting my post-bac, so I think my application is a lot stronger with the extra year.
A slower schedule also means a lighter schedule, so I can focus more on each class to make sure I get A's, and will have more time to dedicate to crushing the MCAT.
If you're a person who has always had medicine in the back of their mind, has a strong GPA going into this, and already has a lot of the EC stuff covered, then I think a formal program makes a lot more sense than it would have for me.
There's also the fact that some people will benefit from the structured environment, whereas some will not. Some people find an MCAT study class to be really helpful, and some find it to be a waste of money. A lot depends on you as a person/student.
