I figured the OP was trying to decide between which connecting flight from ORD to take: JFK-MAD on a 757, or MIA-MAD, DFW-MAD on 763s...
I've never flown a 757 TATLy, but I have enjoyed many trips on 763s in business.
But the OP asked about Westbound flight from MAD..so why look at flights ex USA? Also if the OP is looking at Connections from MAD to ORD why was the 777 via London not mentioned?