We are going to switch from an homemade framework to Symfony or Yii. The capabilities of these two framework are, from our point of view, similar (ie they both have what we need).
Our situation : php 5.2.9, medium-size website (10.000 uniques / day) but shared hosting.
Please no answer like "this one is better", ideally someone that would have use the two frameworks recently.
symfony2 is not out yet. it's no more preview but its still beta.
to point 2; depends whats your limits in php (e.g. memory limit) symfony has by the way a nice caching strategy, so it shouldn't be a problem.
as you mentioned, theres no answer like "a or b is better". but you should check if you are able to run those frameworks on your shared host (sometimes its a bit bitchy to set up symfony on shared hosts).