I have only flown the Mayhem and the Showtime, both with Saito 1.25's. My impressions are the following:

Mayhem: For sale, less engine (putting it in another Showtime for a back-up).

Showtime: Keeper (Remove engine from Mayhem for back-up Showtime).

There is no comparison. They are very different planes, but if you want to have a plane that will do any 3D or pattern for practice, get the ST.

The Mayhem is still a GREAT AIRPLANE, just not for advancing your skills.