Richard Jones did a nice presentation about this in August 2011, comparing 10 Python frameworks along a lot of useful metrics (perf, simplicity, size, documentation, etc) by developing the same "hello world"-ish app in each one. He even scored them at the end.

The slides are at http://www.slideshare.net/r1chardj0n3s/web-microframework-ba... -- they don't seem to be loading inline now but the PDF download still works. Video of the talk is at http://www.youtube.com/watch?v=AYjPIMe0BhA .

Spoiler alert: the winner was bottle.py, by a nose.