I'm the proud owner of on of each. The GGTS, with the AVS, offers a lot of flexability with atty and battery combo's. Great looking mod to boot. Using one of TW's 510 battery extensions, you can even integrate an eGo tank without the hideous gap. The purely mechanical design makes it almost indestructable. Keep a good shine on it, and with the GGTS you'll be the life of the party! The GGTS also allows you to swap between different model attys, too.
The ProVari doesn't have the flexability with batteries that the GGTS has. It is also considerably heavier than the GGTS, even with the shorter battery. Good rugged design, the VV makes it a very solid all day vape as well as a good specialty vape. Works well with most 510 attys and most carto's.
As you can see, I had the same question. The only answer was, to get them both. Each has it's nich in the day. With the MAP, both rock (although you'll need an extender for the GG). I usually carry both with me, and either a slim or an Inferno on any given day.