Not at all. The only real reason I would see is F/I options ? It's quite clear you can F/I a GT350, and not worrie with the car going boom. Also the GT would need several proper upgrades (depending on options purchased) that the GT350 already has. So nope. Not at all.
I test drove a GT and thought is was a very nice car. At the time I was aware of the GT350 and wanted to get that version due to the engine, brakes, transmission, suspension, and cooling upgrades.
I ended up getting a pretty early 2016 and now have about 35,000 miles.
Never considered a Shelby b/c my budget was $30K or less and and ended up getting a new GT for just under $29K. I have zero regrets getting this much performance for what I paid.
Never even considered a Ford(Mustang GT's included) until the GT350 came out. I'm sure the GT is a great car, but I'm all about high revving NA engines and GT350 with the V8 is the only one, and absolute king in the sub-$100k category.