Great topic!
I think the main question is what period tone you're after. Most people will hear the mid pickup strat tone and think "that's a Garcia sound" but that's just one of his tones. The SG isn't the sound most hear in their heads when they think of Garcia's sound. Typically I think folks associate him with tiger and the McIntosh.
I love that early, hopped up tone he had before things got all squeaky clean quite a bit. I think the next guitar I buy with be an SG rather than a tiger/wolf clone. Sounds perfect on St. Stephen and some other tunes from 68-70.
I currently gig my tribute band with a strat and it does the period I like the most (72-74) really well. I'm not sure about when the super 2 and dual sounds started being his go to pickup (78?), but I believe he was running singles for a while in the wolf...I like that sound a lot too, though it's tone (and tiger) pretty sterile especially the overdriven tones. I attribute that to the Mc being used over the vacuum tubes in the twin heads.
The amp is a huge part of the tone choice...not the pre, but the power section. Early Garcia had the power section worked up pretty hard. That and the humbuckers going into the front end of the amp give it that heft and singing overdrive. When you shift to lower output single coils...like on E72, you get a little dirt, but mostly clean.
The strat, using the power section of the amp is my favorite tone. It's not the super clean, heavy, thick tone of tiger into the Mc with dirt pedals for overdrive, that's for sure. I never have gotten along with the hippie sandwich guitars all that well. The Travis Bean had a cool sound too, but I'm at a loss of how to describe the tone...plus there's a boogie during that era so there's les of elements to chase down. I just own a strat and a few blackface amps and like that nails the tone from the period and lineup I like the best...so I went with that.
The SG, unless you go with a three pickup model, will not do the latter years Jerry thing very well IME. But it'll rip up that early stuff perfectly.
I only gig with the strat...if I had to have only one, I like the strat the best.
Now I'll go on eBay and see about a nice SG!
J