I've built and flown both the Great Planes and the Goldberg Extra kits. I no longer have the CGM Extra but I do have two GP Extras. One has a St75 and the other has a ST90. Both fly great. I've got more time on the one with the 75 and it has unlimited vertical and will hover around 1/2 throttle. I have yet to weight it but I'm sure its under 7.25 pounds. The other one has only been flown 5 times and the engine is giving me some fits. It builds really well and the manual is pretty good.

The CGM Extra I built a while ago so it was an older kit. I can't comment on how well they build now. But the old kit didnt build to well. Mainly it was the die cutting. I thought it was horrible. And the plastic parts aren't worth messing with. GP plastic parts are better but I'd still go with fiberglass parts on either. I powered the CGM Extra with a ST90 and it flew awesome. I wasn't in to the hovering and stuff I am now but it did everything that I could throw at it and then slowed to a crawl for landings.

Either way I think you'll like them both. The GP is cheaper and lighter but is also a little smaller than the CGM Extra. I've heard that CGM kits build better than they used to and still fly great as does the GP Extra. Either way I think you'll be pleased. I have been thinking about getting my old CGM Extra back or buying a new one. They both fly like bigger airplanes.
The 91FX should fly both great but the GP Extra will have better vertical performance than the CGM because the CGM is heavier.

Good luck!
Shane