I know this is an old post but I'll voice my opinion since he asked. I'd vote for the twin turbo eleven times out of ten.
If I'd have thought about modding a Challenger (performance wise), I'd have bought the 5.7 and put a Hellion on it. It'd have better than 6.4 power levels and the option to control power with a boost controller (E-Boost 2). The 5.7's take boost better than the 6.4's do but the down side is that my draw to our SRT was those Friggin' garbage can lid sized brakes! They look and perform so darned good! The other features of Ruby were also draw but she was just the total package.
My Wife wanted the SRT over the Scat Pack and even the Hellcat because you couldn't get a roof in a Cat in 2015 and also I'll NEVER pay ADM (or even sticker) for a car that's going to lose money before leaving the lot.
Back in 2015, when the updated body style came out, we also may have gotten one of the best deals around too (I'm cheap and our dealership pulled a Godfather on us..........They made us an offer that we couldn't refuse

)!
I hope that the OP posts up what they decided to do because I'm a little curious and also...Sorry for going so far off track and ranting like a Knucklehead!