One thing of note is that 75L's (the kind of 757s with NGBC seating in Business have their own designator 75L) are only used on shorter flights. Which means less sleep time. If you're starting out from the middle of the country (like the OP from ORD), some might find DFW-MAD preferable eastbound simply because it gives you more time to sleep than JFK-MAD. (DFW-MAD flies reasonably near JFK along the way, and it takes it hours to get to that vicinity.)

For this reason (as someone based on the west coast), I would not tend to choose a 75L to a place where there are other options.

And if I had to fly a 75L, I'd try to eat before the flight (easier if you have long connection) so that I could skip the meal and get right to sleep on the 75L. (In fact, that's what I did the one time I flew the 75L, when my DFW-MAD flight had a technical problem and diverted to JFK. AA rebooked me on the 75L JFK-MAD the next day, but that meant I had long connection time, and indeed spend many hours in the FL, and found the food there sufficient, given the freeness , to eliminate the need to eat dinner onboard the flight.)

Obviously, this would tend to be a consideration only going eastbound.

Finally, I prefer not to have anyone climb over me or for me to need to climb over anyone (especially while the other is sleeping). That's impossible on the 75L, but you have at least a 33% chance of it on the 763 with randomly-assigned seats, and a much better chance if your upgrade clears in time for some D or G seat to still be avialable. Though obviously, if you prefer an actual window seat, the 75L wins there, because it's got 50% window seats, while the 763 has only 25% window seats.