Never used a cache like this before. The problem is that I want to load 500,000 + records out of a database and do some selecting/filtering wicked fast.
Judging by their releases page, OSCache has not been actively maintained since 2007. This is not a good thing. EhCache, on the other hand, is under constant development. For that reason alone, I would choose EhCache.
Edit Nov 2013: OSCache, like the rest of OpenSymphony, is dead.