The Sig is a bit smaller, and without doubt the more complex of the two to build. Although, from what I've read about building the GP, if you want a building project either one will keep you occupied for a while...

I have flown two GP SkyBolts; I actually prefer the Ultimate. One of the two was definitely overweight, and a handful. The other one was quite decent and very pretty in the air; enough so to convince me that I wanted one.

I've got a NIB Sig SkyBolt kit, with a newly-purchased extra set of plans, laying idle. PM me if you're interested. My interests have changed.