I can speak to this, as I use Prelinger quite often.
MPEG2 is "richer." More info. Granted your processor works harder to crunch the billions-more "oh's" and "ones," but, but it's worth it. MPEG4 is highly compressed.
And, regarding "conversion..." I've had good luck just ingesting it straight into FCPX. It transcodes / optimizes. And I, personally, haven't had any problems.