Unless the 757 is one with lie-flats (which you don't indicate so I would presume it's not one with lie-flats), the seat in F is going to be the same. I wouldn't pay $200 more for a standard 757 over a 737-900. The only slight factor to consider is how $200 relates to the time each departs and how that impacts your schedule and what $200 is worth relative to your time.