A320 is a FAR superior place to spend time. Not even close. A320 flying at present is usually 2 legs a day, far fewer all-nighters, and generally much more pleasant. The 737 does Hawaii, but if you're junior you won't see it much. TONS of all-nighters, more Latin America and Carribean, and longer legs. On the plus side there is more variety on the 737 because it's twice as large of a crew base.
The thing about the flying is that it can change at the drop of a hat, except the A320's won't ever be headed to Hawaii.
I've flown both out of SFO and to me the A320 is much more desiresable.