AA facing flop raise from 15/11

[AZplaya]

Villian is 15/11 over about 260 hands. Type of player you don't even notice is at the table, plays very straight forward. I had trouble with this because obv a set is in his range but I think there are some overpairs in his range as well. Thoughts?

Full Tilt Poker
No Limit Hold'em Cash Game
$0.25/$0.50 Blinds
5 Players - (LegoPoker Hand History Converter)

SB: $54.55
BB: $11.40
UTG: $88.40
Hero (CO): $59.35
BTN: $56.25

Preflop: Hero is dealt A[img]/images/graemlins/spade.gif[/img] A[img]/images/graemlins/heart.gif[/img] (5 Players)
UTG calls $0.50, <font color="red">Hero raises to $2.25</font>, BTN calls $2.25, 2 folds, UTG calls $1.75

Flop: ($7.50) 6[img]/images/graemlins/diamond.gif[/img] 4[img]/images/graemlins/club.gif[/img] 3[img]/images/graemlins/diamond.gif[/img] (3 Players)
UTG checks, <font color="red">Hero bets $6.00</font>, <font color="red">BTN raises to $15.00</font>, Hero???