What language (Java or Python) + framework for mid sized web project?
As many things in life, this depends on what your goals are. If you intend to learn a web framework that is used in corporate environments, then choose a Java solution. If not, don't. Python is certainly more elegant and generally more fun in pretty much every way.
As to which framework to use, django has the most mindshare, as evidenced by the number of questions asked about it here. My understanding is that it's also pretty good. It's best suited for CMS-like web sites, though - at least that's what it's coming from and what it's optimized for. You might also have a look at one of the simpler, nimbler ones, such as the relatively new flask. All of these are enjoyable, though they may not all have all features on AppEngine.
I plan to start a mid sized web project, what language + framework would you recommend?
I know Java and Python. I am looking for something simple.
Is App Engine a good option? I like the overall simplicity and free hosting, but I am worried about the datastore (how difficult is it to make it similarly fast as a standard SQL solution? + I need fulltext search + I need to filter objects by several parameters).
What about Java with Stripes? Should I use another framework in addition to Stripes (e.g. for database).
UPDATE:
Thanks for the advice, I finally decided to use Django with Eclipse/PyDev as an IDE.
Python/Django is simple and elegant, it's widely used and there is a great documentation. A small disadvantage is that perhaps I'll have to buy a VPS, but it shouldn't be very hard to port the project to App Engine, which is free to some extent.