If cost is at all a factor id say get the 105, I have had mix of 105 & Ultegra on my road bike vs Dura Ace on my tri bike, I honestly cant tell any difference between any of them, even when components are older and more worn you hardly notice.
What is a factor for me now is that im toying with changing the cassette on my tri bike to a easier range (12-25, maybe 27 instead of the 11-23), its about £120 for just a cassette. I guess im more cost sensative now than I once was, especially as im not aware of the difference, that said im not going to put Ultegra or 105 cassette on a Dura Ace groupset so ill just have to swallow the cost.