Actually it's not. I would be more senior as an 737 FO in DCA than I am as a 320 FO. Additionally my seniority the last six months has stagnated somewhat as DCA 737 FO's bid over to the Airbus. Also after this last bid the most junior DCA 737 CA is quite a bit junior to the bottom DCA 320 CA.
FWIW if I was living just south of BWI and there were no 320 slots available in DCA I would bid 737. I know the aircraft gets a lot of hate here but I flew it for three years for a prior carrier. It's not that bad.

EDIT
I didn't see that an another poster had commented on this. Didn't mean to pille on...