Trumpets play in mvts. 2, 3, and 6. 2 is in Bb, 3 is in D, and 6 is in C.
Personally, I'd prefer C trumpet throughout. Rotary might be fun, but unless the 2nd player also has one, there's probably not much point. Chances are good you're just going to be told to play everything softer anyway.
Have fun!