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.
It's worth noting that he's mainly concerned with publishing existing python apps he's already written as simple web services, not writing a complete web site from scratch. If you just look at the slides it's easy to miss this. This doesn't invalidate anything he says as such, but it's worth keeping this in mind if you're planning on doing a more complete web site.
Despite Jones' use of standardized metrics, in real world use of three of these frameworks I found that his scores indicated little about how effectively each performed.