web.py and flask
Flask is a new and a very active project with good documentation and guidelines for new development and plugins. Its community is quite large and Armin (the lead) is an A grade programmer. Hence it is definitely the better contender. 
Web.py is an older and more mature project which is more a library than a framework. It's what we use for the Internet Archives Open Library website. The documentation is spotty but the code is rock solid and it's over the years accrued a lot of useful utilities into it. 
The final decision is yours but for a new project, I'd go with Flask.
For these pythonic web frameworks; 
They looks similar, except a few minor differences like their url handling schemes. So it is very hard to choose one of them.
I want to learn from your experiences that which one more suits for a web application that is small in size, but need long term support (initial application is small but it is supposed to became larger over time) and why?