I have not played the two sousaphones back to back. I have played two 40K's which were each very fine sousaphones. But I own a 4 valve Martin, that when I first played it, seemed to me to be one of the very the best sousaphones I had (and have) ever played. It's very easy to play, with solid pitch, and a warm sound.
You probably wouldn't go wrong with either instrument, unless you have very specific needs or goals for the horn. But if you find one of the Martins, I wouldn't let it get away.
Mike Lynch