In the case that you can't confirm BF seating, then I would take the -700 to OAK and take the extra bonus miles. I thought the F/C seats on the -700 were as comfortable as any of the 737 series seats are, plus they have wings, whcih not all F'C sets in 737s have, plus they are probably pretty new, versus the F/C seats you are likely to find on the 757.