I've a big huge site..megathemed...it makes life a lot easier for me.(all inner pages and subdomains are pretty well optimized for what they are there for).but it's depend entirely on your niche and what you want from it though.
Make it one site - that way it is easier for Google to delete your site from their index. Certainly much easier than if you have 4-5 sites - it will be that much more difficult for them to make a "mistake". Think about it that way for a moment...