from UTG perspective, this is a spot i like to bet my draws, just about all top pairs and overpairs. it makes it really hard to tell what i have, and puts slightly better hand holders into the tank, as is seen by rocky's difficult decision. (if i get raised back hard, well, thanks for all of that information, guess you have a hand sir) for this reason, this is a good spot to bet all of those types of hands strongly, especially w/ 4 to the flop. he might just have TPWK since he got to call last PF w/ such great odds and just wants to either take it down right there or get it down to one opponent.
i am with 'rilla in that a really big hand probably checks from that early on, looking for a checkraise or call/lead turn line. so after consideration, i pop it up to about 45-50.
comparing calling and raising, i think raising is best, although it's a subtle and not obvious difference.
if you call:
-you still face the possibility of another OOP raise from UTG on later streets - if you are going to call the later raise again, then raising here is better, and if you are going to fold to that possible turn/river raise then folding/raising here is also better
-you allow CO and BB much better odds to call behind you on a draw
if you raise:
-you probably chase out any draws from other opponents, possibly UTG as well
-you may get a call from a worse hand, and this likelihood decreases as more scare cards are allowed to fall on subsequent streets
slightly better hands calling/folding to your RR are non existent in this spot. if you raise it's probably going to be raise/fold from villain and that reaction would probably be accurately indicative of villain's holdings compared to yours.