Here is a pic of my GMT-Master IIc next to my Sea-Dweller. I think the Sea-Dweller is the same size as the regular Sub you are talking about. Hope it helps
I have the GMT-Master IIc and absolutely love it, my friend has the Daytona and it wears smaller for some reason still a very nice watch looking to get it next myself as far accuracy I believ they are both excellent replicas out of the box with no mod's needed for me, I could not pic a best base on accuracy. On the other hand you have the movement and according to what I have read the Daytona is less reliable than a Swiss ETA ICHS (incorrect hand stack)that you can get on the GMT, so movement wise I would pic the GMT. As far as the dealer it depends on your location. Where are you located?
Try this set them at all hands at 12 pm then put the crown back in after its start running then pull the crown all the way out and forward the time for about two days that is what I did to one of my gmt omegas and afterwards started keeping track as normal. I do not know if it will work with the one you have because the handstack is different on the omega I think.
So the hour hand ran for 20 hours from midnight to the 8 pm position while the GMT hand only traveled 12 hrs to the 12 o'clock position on the bezel is that correct?
If that is the case the GMT hand lost 8 hrs or gained 16 hrs.