I was a Sr B727, moving to Jr B757 about 10 years ago. I would ask are you single, do you commute, do you care about destinations? Even jr trips to UK, and DOM on the B756 are good for commuting (plus I like the plane better), but above the splat line on the B737 might give you some choices but still, not very commutiable.
i'd lean B756 for myself, good luck.
I definitely like the 756 better as a plane, but having a wife and baby and home, specific days off that match up with my wife's work schedule are very important, more so than money, destinations, or equipment.
I'm thinking I would have more control to get M-W off every week on the 73. Also the 756 has those ridiculous schedules where you work about 10 days straight sometimes.