Who made the bigger mistake?

[Baltimore Ron]

The Scene: Party Poker 10-20 - very loose and not very aggressive. The other table from the one in my post "Save 1 big bet?"

I get dealt A3o on the button. The utg (our antagonist in this hand) limped, a couple (three?) others limped and I limped while the blinds came along. Loose I know, but I'm feeling spunky at this table and feel like I can outplay these guys on later streets.

The flop was K3x. After the utg bet and the others folded I raised to get heads-up, which I did. The utg called.

The turn was some medium blank card and we both checked.

The river was another 3 and the utg check called my bet and showed K9s (or was it K8s? I don't zackly 'member.)

BR