Ive got one

The fourth pointer measures time on the 24 hour clock around the face of the watch which helps you track a second time zone. You can rotate the bezel to offset for hours deviation from GMT too.

Service costs for any rolex arent cheap (guess at 400-500 every 5 years) but its a robust watch and mine has taken a load of abuse. Runs about +2 seconds per month so accurate (for a manual watch!)

If its a toss up betwen a submariner and GMTII key difference are

GMTII can measure a second time zone
GMTII has a larger case than submarner with wider lugs which can be a visual issue for some
GMTII has ceramic bezel
GMTII has shiny centre band on strap which some love, some hate
GMTII its a heavy watch, slightly heavier and bulkier than a sub which may make it harder to wear under a shirt cuff
GMT "only" water resistant to 300m (think standard sub is 300m too but sea-dweller is 1200m and new deep sea is 3000m)

try them both on and see which you prefer. Both are awesome watches.