ORIGINAL: Missileman

HDX is not a T-Rex but another 450 size heli distributed by Helidirect.com , a Nice heli though.
T-Rex made by align has the XL, SA, and several SE models.
XL is all plastic but a nice heli and great place to start.
SA has metal sides and many plastic parts in the head and tail.
SE is top of the line and has carbon fiber frame and mostly metal head and tail.
HDX (not made by Align) is built on what is known as a super frame (some T-Rex owners upgrade to this frame) made of G10 fiberglass and has full metal head and tail. Some T-Rex parts are compatable but not all (I had to replace the feather shaft on mine and none of the T-Rex shafts would work but the T-rex main shaft and main gear set does fit)