There's always a reason to have another heli
My main reason for buying the HK-500GT was that I didn't have anything larger than a 450, and after all the upgrading and swapping of head parts with the HK-450, I was hoping the HK-500GT kit would not only give me that larger, more stable heli, but also parts that would not need upgrading down the road. So far the HK-500GT kit has lived up to and exceeded my own expectations.
All of the "upgrade" HK-500GT parts on the HC/HK website (with the exception of the carbon blades and frame), are *already* included with this kit. The head is mostly metal, and the tail assembly is mostly metal also. The nonmetal parts on the head and tail are FRN (fiber-reinforced nylon), so I don't foresee anyone needing to go out and drop $$ on upgrade parts. The good news is that HobbyCity already seems to be counting on the HK-500GT as a top seller, so they have a good stock of replacement parts already, most of which are 1/3rd (or less) of the cost of comparable T-Rex 500 parts.
I've seen the plastic version of the HK-T500 (a guy in my club bought one RTF), the metal head and tail assembly alone on the HK-500GT make it well worth the extra $16. I can't see why there would be a reason to buy a HK-T500 over the HK-500GT to save $16.