I have not run this through the computer, so you may be right, but there are a few reasons that I didn't take back with the pawn. Firstly, taking back with the pawn sacrifices part of black's initiative. Alternately, I let white use up one of his moves capturing instead of doing something more useful. Secondly, taking back with the pawn gives white an open c-file. I believe that giving back the pawn is worth keeping white in an disorganized position. Finally, taking back the pawn relieves white of the duty of defending his weak c-pawn. If 13. ... cxb3, then 14. cxb3. IMO this gives white a significantly more active position. 

Slick second game; nice knight work in the mate.

In the first game, would 13. ... cxb3 avoid you giving back a pawn?  After the take-back with a or c, you could castle anyway.  Is the worse pawn structure worth abandoning the pawn?

Gotta admit it took me a bit to see why Bf3 was sweet; I probably shouldn't be playing today...