Multi-DB versus W3 Total Cache Dilemma?

I read the post that discusses a little bit about how you might be able to combine these two but I'm rather reluctant, especially as my technical expertise is quite limited at this time.

What would be the best situation for me if I had to choose one or the other?

I plan on having around 400-500 blogs running with my buddypress site. Furthermore, I expect the community component of my site to be extremely active because of the nature of the type of website I'm developing. So a couple of questions come to mind... Does the Multi-DB plugin distribute information out only for blogs or does it also distribute things out relative to the buddypress community components? If it only distributes by blog, does that mean the entire community aspects run on a single DB? With all of these considerations in mind, would Multi-DB or a caching plugin like W3 Total Cache be the better way to go?

Is there a good alternative to W3 Total Cache that works extremely well with the Multi-DB plugin?