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.