The problem with VNTs are two fold.
High exhaust gas temp seems to fuck them up too quickly.
Setting up the VNT mechanism to work ideally for your car (if it not the standard car the turbo came off) can be a pain in the arse.
Lots of types of VNT turbos, from teeny T2 sized ones to mahooooosive VNT GT40s which look the tits.
Companys like Holset etc also make VNT turbos.


GT turbos are more modern than Garrett T-series ones, but they seem to use thinner shafts in many cases, which makes them slightly weaker.
Most people who has done back to back testing has said to stick with the normal journal bearings too, as the BB cores seem to be weaker, more expensive, and dont seem to noticably help perormance.
And seeing as you dont HAVE to have a BB core on a GT turbo, well...


Many aftermarket turbo companys use a mixture of components, GT, T, and their own designs, to make their ones, so something being called a T or GT summat if its not made by Garrett dont nececerily mean loads.


Working out sizing of turbos is down to wheel and housing sizes compared to engine spec (mainly capacity and working rev range).
But far too long winded to talk about here, when can read about it all over internet.


Give a detailed spec of what car its for and what you want from it, and might be able to help.