What are you going to fill it with, and what is the starting charge pressure for each gun.
FX T12 Whisper: no pressure gauge, expensive options , no air adjustments, plastic stock but nice. Can you get parts?
From what I have read on various forums, the .177 clip can damage some pellet skirts.
The .177 accuracy can suffer until the barrel support is removed.
Rainstorm: not out yet, no reviews, Too hot for .177, sonic boom shots from a very quiet gun? how will you slow it down?
Marauder: entry level, decent value for $400 AOA deal, not as nice as the other 2, might have to visit Greg Davis to shoot good for $150+. Talk of problems and returns in the forums.
I think the Mrod might be one of the better choices for .177 because of the adjustability.
I was really thinking about the Blizzard S10, but it also may be too hot for .177 without a weaker hammer spring. With a little more research, I just might click one.
Just my 1.5 cents
Rick