This is a tough decision as I've been confronted with it myself.

My $0.02 -- upgrade on the 737, switch over to the 320 later. You're a first time captain upgrade with the company; you're already typed on the 737 and going 787 --> 737 will be a cakewalk. You'll benefit from the better seniority and movement. If you're still inclined to switch over to the 320 a few years from now, do so when you have better seniority. However, if you think you think you're going to inevitably go Airbus, maybe do it now, 'ripping the Band-Aid off' so to speak. Flying the jet is easy, but learning to translate Boeing into Airbus has a little bit of a learning curve.