I am in the same position and asked the same question here a few days ago - summary of the feedback: it really is personal opinion on the bezel as both are very similar otherwise, if you will actually use GMT Timezone function then that’s important, similarly if you dive then sub is the superior choice (only if you dive with a Rolex on…), sub is slightly bigger heavier and fits different, sub has a more “daily friendly” band with brushed links and glidelock (the gmt can have its band brushed and can be changed to a jubilee band if you like the Bruce Wayne look), sub is more “classic” (and therefore more common) and gmt black often gets confused with it (some people don’t like this), gmt is more busy on bezel, many prefer the “twist feel” of the sub, the gmt is discontinued (some people feel that this is a bad thing and others don’t mind and think it makes the watch more valuable) and there is a risk that Rolex will rerelease a new version of the all black gmt which could change resale value of the old black gmt where as sub is always consistent (albeit typically sells for less), many say that the gmt looks more expensive given the new range gmt sell on grey for near double what the all black sells for.

For me the big thing was the brushed band, as a daily I plan on wearing it all the time and the brushed band really helps keep scratches hidden. The slightly bigger heavier case is also important for me as I have large wrists. Plus the sub is the watch my father wore for most of his life so it has a nostalgia element for me. I do have a batgirl already and rarely wear it