My understanding is that chorus and ultegra level components give you the biggest bang for the buck, in terms of quality, performance and weight vs price. You pay a lot to get a little more w record and durace, but then I think you lose a lot ore for the price savings when you go down to ultegra, at least that's what the fellow that built by custom crank set told me (and he is a biomechanicist for the cycling stars).

Given that you are springing for another custom bike, I would guess the diff in price between 105 and ultegra is a small percentage of the total bill on this bike. If it were me I'd go ultegra all the way, or for you if you really like campy shifters ultegra/chorus. I think you'll enjoy it more, especially in the mountains. If you wanted to save $ you could have had your present bike retrofitted with s/s couplers. I did that and was very pleased with the result (bilenky did it for me).