Make sure the bike has full 105 or full Ultegra. For example, Bikedirect has a titanium bike that says "Ultegra 6700" but if you look closely it has an FSA crank and Cane Creek brakes.

As far as 105 versus Ultegra, I've had good experience with both. The only non-crash problem I've had with either is a broken 105 shifter and that was after 11K miles. That was 9-speed, not the newer stuff.