I have the GGTS and the GGTB. I actually prefer the GGTB over the GGTS, because I really only use one battery combination. That said, the GGTS is a great mod for the following reasons: It is all mechanical - No electronics or switches to break. It works every time you pick it up. It is adjustable, so you can play with different voltages and use the batteries you already have (if you already have some). It uses 801,808, 901 and / or 510 atties and carts, so you can use anything you have on hand, and experiment with all the latest atty gadgets. It is very well constructed, and if maintained, is a showpiece.
The downsides are the size (it's like a cannon compared to a mini), the finish (it looks great, but I have "acid hands", and have etched the surface off of my nickel GG and corroded my aluminum one), and the lack of VV.
The AFS / UFS is a cool option, but it is "advanced". I have the AFS 1, 2 and 3, but not the UFS yet, and when they work they are great, but they are fiddly, and I usually end up going back to dripping, rather than mess with them. The 5V passthrough is nice, but you can't get them anymore (except on the classifieds)
When I got my GGTB, I stopped using my GGTS completely. I just got three batts (1 in the home charger, 1 in the work charger, 1 in the mod) and a few 901s / 808D-1 Carts, juice and a drip tip, and I had everything I needed. If you are into gadgetry and like fiddling, go the GGTS route, it's much cooler. If not, the GGTB will work just fine and do everything you want.
All that said, I'm vaping on my Darwin at the moment, and I haven't picked up my GGTB since it came.