A question came up on linkedin the other day and it is one which I’ve seen come up quite a few times on Twitter and in Stack Overflow
Which CSS Framework Do You Prefer?
Not sure why though, probably because it was the first responsive Framework that I used.
Twitter bootstrap is much more popular, but I’m putting that down to the fact that it is more widely known and associated with Twitter rather than it being a superior tool.
At the end of the day they are both great tools. Personally I think they both put more code into a project than needed for production, plus you are kind of bound by their own limitations unless you want to rework a lot of the core rules (at which point you would probably just start from scratch).
Having said that I’ve used them for production projects before due to client budget restraints and I always use them for mockups and browser based wireframing and UX testing.