Submariner or GMT if you had to choose one

[Chrislec]

I think you are not asking the right question, because both are great watches (it's like hesitating between vanilla or strawberry).

I'd suggest to look at yourself in perpective or in the mirror, think about what type of guy you are in your life: Are you a Magnum or a James Bond type of guy? Easy going or casual-sophisticated? Then the answer will naturally come to you, because the watch will better fit the personality you think you have. A little bit like a mirror of yourself.
At least it works if you had enjoyed both TV/movie series in the 80s and you understand the iconic aspects attached to them.

You may realize you need both because your life has moments where both characters would match... So which one to pick first?

Hard question indeed. Already 120 answers to your post. Because it's not a watch question: it's a personal psychology question.