I have both and IMO the TRex is the better all around buy. The X 400 is fine, once you replace the weak head and figure out how to get rid of the elevator slop.

My 400 flies very well but I have gone with an aluminum head, aluminum swash, bling tail rotor grips and two sets of landing gear.

Also not all parts from Align will work on the 400 and vice versa. The main motor gear set is not the same, nor is the landing gear.

The 400 is attractive because if the price and because it's an ARF but the TRex is not hard to build and has a better all around control system.

IMO..