When one of my rotors was fouled with hydraulic fluid I upgraded both to Ultegra. They work very well and the center lock design makes them very easy to install with just a cassette tool and chain whip.
Replaced OEM Tekro calipers and much prefer the braking action, feel and looks. Install was straightforward, they do fit Conti GP 4 Season 28's. take your time adjusting them, and you will be rewarded with a firm, responsive feel and great stopping power.
The Ultegra is the 105 caliper with a stiffening frame added, not sure there's much difference, but given the minor costweight delta, the Ultegra seems worth it. And they look good.
When it comes time to replace my chain, I go between Dura-Ace and Ultegra depending on what's available at a good price. I have been extremely happy with both and wear seems to be about the same. Just keep it lubed (about every 300 miles) and wipe it down after every ride.
