When I was building new wheels a few months ago I also considered the race and strong. I bought one of each and stuck them through the Shimano hub flange, and the strong needed some force to bend the spokes around the flange for the ones with heads in the inside of the flange. So I went with the race.

The thicker strong would stretch less than the thinner race, but I have very short spokes on my 20" wheels, so the amount of stretch with the short race on 20" would be less than a long strong on 26".

I assume that the double butted would reduce wear at the nipple compared to single butted. But the thicker elbow on the strong might be appropriate in some cases for heavy loads.