The differences between the vehicles in a literal sense is relatively small. They both look different, have upgraded suspension, and more power via supercharger. So in a way, they are very similar.
I am not familiar with the R tune package. http://gt350.shelbyamerican.com/forms/2012GT350sm.pdf
But in my mind there are 2 things that are pretty different between the Shelby and the Roush:
1. Appearance and your preference of what Shelby does vs what Roush does.
2. Your opinion of Shelby vs. Roush. Some people would sell their left lung for a Shelby anything. Others think it's just not worth the pricetag. I know of other people that think Roush is very very cool and would jump on that name. It's really up to you.
But in the end, there will probably be a lot of changes between now and Jan 2012 that will pretty much make this decision moot for now!
Why the "turnkey" tuner infatuation?
2005 Ford Focus ST 2.3L- Focussport Intake, Cosworth Intake Manifold, Cosworth Header and Flex Pipe, Trubendz 2.5" exhaust, FSWerks Tune on X-Cal 2, Hawk pads, stainless brake lines, Quaife LSD, 4.06 Final drive, Exedy clutch and flywheel, KW Variant 1 coilovers, Steeda STS shifter
On a Mustang Forum for fun....