A Predator is nice but I don't think it's as quick and nimble as you are looking for. I haven't heard of any problems with the nitro Trex. The elec. will provide more power than the nitro version and who ever told you it's not a hot rod on 6c must need a hell of alot of power to fly a heli. I've flown one with 600mm blades and thought it flew very well and didn't bog down for my flying. The owner would sometimes bog it down, he put some 550's on it and it is ballistic. I don't think the nitro version made much news because it's basically the same heli anyway, plus there are already a ton of 50 size helis so it's really not a history making event to release a 50 nitro. theres not that many 50 size electrics. You just need to pick your power source, I'm sure you'd be happy with any TT or Align 50 size heli.
Hey maybe you were thinking of the TT Raptor 620e that lacked power on 6 cells, it does! It's a little larger and heavier than the Rex and needs a little more juice.