jucava,
I own both the UCD 46 w/a saito 100 on it and a mayhem with a YS 120 for power.
I am very happy with both the planes, however, here are a few differences you might want to consider. NOVAflier is right on describing the performance of the 46 UCD, but it's a great plane for the 3D beginner. The mayhem has some advantages over the UCD, one being the obvious, it's bigger, and it's more stable in the wind.
The quality of the Mayhem is better, especially the covering and the hardware that is provided. Mayhems rod linkages are 440 not the 256 size.
For me the Mayhem performs the majority of 3D maneuvers, at least in my repertoire of maneuvers, easier than the UCD. And I still have not been able to get a decent knife edge out of the UCD no matter how far I move the CG back, however, with all that weight in the back the hovers are almost automatic. The one thing the UCD has way over the Mayhem is rolls, man on high rates they are blinding. That and the dual elevator servos in the rear, gives you some other options when using a computer radio.
The Mayhem handles high speeds better than the UCD, when you start getting any flutter from the tail surfaces on the UCD back off quick because if you don't the horizontal stab will come apart. Of course 3D planes are not built for speed.
Fortunately for you, the altitude you fly in will not factor in as greatly as it does for me, you will get great performance from smaller engines, although I think the saito 100 would work great for you in either one.
I believe you will enjoy either one, but I give the nod to the Mayhem.
Dhammer