the tektro brakes in question are of the sort where the arms stick out almost horizontally. The shimanos in question are of the sort where the arms are more diagonal. If you're familiar with Paul Components Engineering's cantis, the tektros are like the "neo-retro", and the shimanos are like the "touring". FWIW, Paul claims they made the tourings in response to demand for a center-pull canti that was more "bag/rack/calf" friendly. (from the paulcomp website:
http://www.paulcomp.com/ ) The neo-retro came first, in response to the demand for cantis made by the cross craze back at the turn of the century. These brake harder. However, and i've never used cantis of this sort, the cantis with the better mechanical advantage may have some issues with typical touring goodies, due to their location.
FWIW, I have the shimano br-r550s on my 26"wheel LHT, and i like 'em. I had to use a crazy tektro straddle cable to clear the fenders, but they work great. I've used tektro oryx and cane creek scx-5 cantis in the past (similar design to the br-r550) and the shimanos brake and feel superior to those. I've never used any of the other style cantis, so i can't give a direct, empirical comparison.
-rob