I would say First is marginally better for food, service, and seat, booze will be a lot better (if that is your thing) plus the CCR in T5 is much better than the lounges in T3 although if you are a BA Gold/AA Exec Platinum you could get in the flounge in T3 which would narrow the gap a bit. So in summary, it's your money so its your choice, personally I would pay the $700 for a long TATL like LAX