The 11'6 is an absolute barge. Unless you're very low in confidence for balance I would be looking at the two smaller options in 11'4 or 10'10. Lay the two down next to each other in the shop, you'll immediately see the difference in height particularly. Better yet, demo one of them.
My buddy's 6ft 5", 100kg with the 11'6 and after a few surfs is already thinking hmmm.....it's a pretty big board...
Re GS vs GT - the GT is prettier. If you don't mind the white and don't mind a single fin setup (most seem to prefer anyway) then the GS is fine.
Read this too.
www.seabreeze.com.au/forums/Stand-Up-Paddle/Review/Naish-Nalu-recommendation/Having said all that, you're gonna be on the water with the sun on your back regardless soon enough, so don't lose too much time agonising over a couple of inches. That's what puberty's for.