How do you choose key?
Well, the first consideration is the accompanist. Since that happens to be my wife, if she says, "No WAY am I playing that thing in five sharps! Are you NUTS?", well, that's a bit of a showstopper.
Just recently I also played "The Lord's Prayer" by Malotte in church. The choices I had were in C major and in Eb major. I chose Eb because, for that piece, most of it falls in what I call the "sweet" range. If it's too low and the phrasing is long, that means I have to drop volume in order to keep from running out of air. It's nice when I don't have to worry about that.
So a second considerations is where the piece lies on the horn.
The actual key doesn't really make a difference to me. If I need to brush up on the key of C# minor to play a fast run, so be it.
U.S. Army, Retired (built mid-1950s)
Adams E2 Euph (built 2017)
Boosey & Co. Imperial Euph (built 1941)
Edwards B454 Bass Trombone (built 2012)
Boosey & Hawkes Imperial Eb tuba (built 1958)
Kanstul 33-T lBBb tuba (built 2010)