Can't really add much more than Vapn, cos he's already pretty much said it all.
Comparing standard Chinese e-cigs with a mod like the GG is not an even comparison really.
In the long run, if you want to save money, mods are the way to go because the batteries are a LOT cheaper than standard e-cig batts and tend to last longer.
Added to this, there's not much that can ever go wrong with a GG apart from perhaps the spring.
With the Grant and the trans and the new TS, you have the ability to vape at 3.7v, 6v and7.2+v (HV atomisers required). Again, no comparison really. High voltage vaping has been a life-saver for many die-hard smokers, but it's not necessarily everyone'scup of tea.
So, having said all this, if you're perfectly happy with the setup that you already have, then there's not necessarily any reason for you to upgrade until you do get fed up with it (it's kind of inevitable that you will).