With a typical driver in an Auto, the R/T is an honest high 13 sec car at ~102 mph. The SRT low 13's at ~108 mph. This is a typical driver, both with 2+ 60 ft. times, street tires, 100% stock. A guy with a lot of seat time can get them around 1.8's and the R/T will be a mid 13 to the SRT high 12's.
I have never driver a Challenger R/T, (have the SRT), so can't say. Can say when I bought my SRT Charger, I did drive a Charger R/T (albeit without the new 5.7), and there was just no comparison. Handling, brakes, power, etc. I got a great deal on my SRT, so the difference was around 8k, and at there there was just no question what to do.
If you are not in a rush...wait a year for the 6.4 and get a SRT