Mongo's killer feature for me is the support. The team has been absolutely awesome about helping me track down bugs and sort out my issues.

By contrast I've spent a lot of time on Tokyo, but I'm actively trying to move away from it these days because there have just been too many mysterious problems. I've tracked down some of them and submitted patches, but it's cost me a lot of time I'd rather be spending working on my product. Don't get me wrong, Tokyo is chock-full of great code, but anything as general-purpose as a database is going to have all sorts of interface and configuration issues that require more time than your typical open-source project founder can put in.