It depends on you.
Personally, I wouldn't want to pack/re-pack after just one night.
If you're going to be in L.A. for just 2 nights, it seems kind of silly to move from hotel to the other. Hollywood is very "urban": old buidlings, metro rail, buses, tour buses, etc.
Santa Monica is a LITTLE less frenzied, ocean breezes, newer architecture, no metro rail, fewer buses. Not as centrally-located as Hollywood but given it's only 2 days, it doesn't really matter.
I'd opt for Santa Monica if you're going to do more relaxing and staying near the hotel. I'd pick Hollywood if you're going to try to pack in as much sightseeing as possible.