I have a Giant XTC SX

I was looking at bolt thru hubs for the back as I am having a wheel built

Will they fit and what are the advantages disadvantages?

You can only have a bolt through hub on a frame that is specifically designed for on, they come in 135x12mm and 150x12mm.  You generally only find these on DH and longer travel / get you into lots of trouble descending type bikes as it makes the rear end stiffer.  So basically you have to have a frame designed for such an axle, and you don't.

 Regular 135mm spaced QR is what you want. (What they'll give you if you ask anywhere for a rear MTB hub).

As is this.

It says it's for DT hubs only but I dont see why it wouldn't fit any 135/10mm hub.