One thing is certain in airline jobs . . . . flying shifts monthly.

The 320 and 737 fulfill similar missions. The flying will therefore shift back and forth with the whims of network planning. The only certainty between the 2 fleets is that the 737 is much bigger and so affords more variety as you gain seniority. Focus on what makes you happy in life and how best to do those things and the answers will be clear: you can maximize seniority and gain the best home life possible or you can maximize position and gain the highest income or you can pick something in between, but you can't pick where a plane will fly or the types of trips it will do (ignoring the obvious of comparing WB to NB).