It depends on what you want. If an independently settable 24h hand is desirable because it allows you to track (and change) a second timezone from where you're sitting, then it's more "proper" than one which you have to hack in order to change the 24h hand.
If you travel a lot (which was the point of the first GMT watch*) then you will probably be better off with one whose 12h hand is independently settable.
* The first GMT watch with an independent 24h hand did not allow any adjustability at all; the alternative timezone was displayed using a rotating bezel. That would not be a "true" GMT by some people's definitions, nor the original and slightly earlier Glycine Airman, which had only a 24hr hand and a rotating bezel to display a second timezone.
Generally speaking, people define as "true" the operating method of the GMT watch they own, or prefer. It's nothing to do with universal agreed definitions (little is, in the watch world!)