What language (Java or Python) + framework for mid sized web project?
I've built several apps on GAE (with Python) over the last year. It's hard to beat the ease with which you can get an app up and running quickly. Don't discount the value in that alone. 
While you may not understand the datastore yet, it is extremely well documented and there are great resources - including this one - to help you get past any problem you might have.
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.