I never saw Queen with Mercury (or without for that matter) and I've never been to a Bowie concert either. Apparently Bowie does "Under Pressure" at every live show - I think that would be epic! There's something about the riff of the song...you hear it one time, and it's all over: the song owns you!
"Under Pressure" is a great duet because it's all rock and no sap. A nice, deep hook and great vocals equal a classic tune.
Can you think of a better rock duet?