[Amon]

Wow, this one is tough. However, I think you played it optimally, but by a very thin margin (flop fold is absolutely an alternative). I don't think you have to be afraid of neither top set or top two pair, as I figure he would have bet at least the pot in those cases. Neither do you usually have to fear A34 with a flush draw here, as that hand warrants at least a pot bet (although without reads you certainly can't discount the possibility entirely). The two spades in your hand are two cards less for villain to fill up a very possible nut flush draw. The biggest risk you are running by peeling one off is getting squeezed by a checkraising set or a big draw. If the 5 had been a 7 or 8 instead, continuing in this hand would feel much more comfortable, but as it is, I am still slightly leaning towards calling flop and pushing any high non spade turn (if it's headsup).