I like the look of the piston horn more. But my GUESS is that the rotary would be a safer pick if you can't test both of them. While it's possible to screw up both rotary and piston valves, rotary valves are easier to make so that they actually work reasonably, and are easier to maintain, because they require less engineering and close-tolerance fabrication than pistons. Keep in mind that historically, the rotary valve was the first on the scene.
For that reason, and absent any other knowledge or experience of these instruments, I'd pick the rotary.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)