What language (Java or Python) + framework for mid sized web project?
Google App Engine + GWT and you have a pretty powerful combination for developing web applications. The datastore is quite fast, and it has so far done the job quite nicely for me. 
In my project I had to do a lot of redesigning of my database model, because it was made for a traditional relational database, and some things were not (directly) possible with the datastore.
GWT has a fairly moderate learning curve, but it gets the job done very well. The gui code is really easy to get started with, but it's the asynchronous way of thinking that's the hardest part.
As for search I don't think it's supported in the framework. Filtering is possible on parameters.
There are some limitations to GAE, and you should consider them before putting all your eggs in that basket. The fact that GAE uses J2EE distribution standards makes the application very easy to move to a dedicated server, should the limitations of GAE become a problem. In fact I only think you would have to refactor the part of your code that makes the queries and stores the data (which shouldn't be much more than 100 lines).
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.