On PR 1.1, MicroB is installed on the rootfs, which is faster than the larger file system where Fennec and Opera Mobile are installed. MicroB is also (partially) preloaded into memory, even when not open. I haven't looked at PR 1.2, so I don't know if any of this has changed.

Of course, I'm one of the guys doing the Opera port, so I'm obviously biased towards making Opera look better than the other browsers.