Both are amazing watches. So you can’t go wrong either way. And, in their current versions, they are similar in size, though the GMT is a few millimeters thicker. (Also, the diameter size is a bit misleading. Given the absence of lugs, they wear much better than you would expect on smaller wrists.)
That said, to me, the double balance is one of GF’s quintessential complications, and they’ve done a remarkable job fitting it into a comfortable, wearable piece, that is simply fun to watch in action. My vote would be for the Double Balancier . . .