Hitachi Microdrive - for the money you pay, you get more capacity, and the effect on battery life is minimal so long as you're just watching videos or listening to music on it, especially if you're using a microdrive mode which caches data in RAM. Plus, I don't trust the Lexar cards due to their varying levels of QC.
I vote for the Hitachi 4GB Microdrive. I've had one since I bought my X50V brand new last year and haven't had any problem.
You'll like it...just make sure it is NOT a generic 4GB microdrive...those scare me. :-)