Hi,
You might wish to review the article about rotary trumpet which is interesting and informative. Others who play or have played rotary trumpets will probably add their opinions as to the differences between piston and rotary instruments.
The Rotary Trumpet: A Voyage of Discovery
By Murray Greig Brass-Forum.co.uk - The Rotary Trumpet: A Voyage of Discovery
You may also want to review:
A Player's Guide to Rotary Valve Trumpets
Best wishes,
Lloyd