Having played and owned both GK and Ampeg gear, there's some things to consider:
Solid state Ampegs aren't going to be much different in tone from solid state GK's, especially the 800RB.
The flagship models of both companies are very different beasts. It's definitely a matter of which tone you like more.
It also depends on your preference for all tube/hybrid/solid state. (Ampeg has all three, GK doesn't do all-tube as of yet)
I'd highly recommend going used as well since you're going to be getting better quality for the money that you want to spend.
Also, the weight is a consideration. I had an Ampeg SVT 4-Pro, which has a particular cooling space need for ventilation when racked. I also wanted a surge protector 1U unit, and a 2U drawer for all the cables. It ended up being a monstrous 140lb rack that took up 60% of my back seat, making me the only one able to ride in my car unless the passenger was a small child. Sold it off rather quick once my old back injury worsened.
All-told, between the two, I'd probably go with either a GK 800RB head, or an SVT-CL and an Ampeg 610 or 810 or two GK 212 or 410 Neo cabs.