My main bike is a single 650GS. It goes everywhere I point it at, but sometimes it's just too much work. My fun bike is KLR250 - light, nimble, with enough power to go anywhere and not enough power to get me in trouble (no spontaneous wheelies, slips, etc). If i were doing the TAT, I'd probably take the 250. If I have to ride two hours to get to the dirt I'm visiting that day, I'd take the 650.
I've seen people on 1200GS doing stuff I would not attempt on the 250. I've also seen people on 250 unable to pass what I just cleared on the 650.
For me, the 650 is about as big of an adv bike as I will ever get. Sure, the KTMs are nice, light, powerful...but all too tall for me. If I can't flat foot it, I don't want 400+ pounds under me in dirt. The 650GS I can flatfoot, even though it is a heavy pig for its class. The 250 I can tiptoe, and that's okay, because it's a light bike.