737.

You'll build seniority faster and can still bid up to the 756 at any time. If you take the 756 initially, you'll be blocked from bidding down the the 737 or 320 for two years.

Bid the 737, fly a few months to get to know the system and your commute. Keep an eye on the 756 schedules that people junior to you are holding then bid the 756 when it looks better than what you're flying on the 737.