The Hellcat is a fantastic car and I'm glad that Chrysler went through with offering their muscle/pony car with more hp than the viper. I have to say that I like the look of the hellcat much more than the '15 mustang. However, the Challenger is a very old platform by now and if it wasn't for the awesome looks and 700+hp motor, it wouldn't be worth talking about.
Even if the '15 mustang isn't as nice looking or available with a ~700hp variant, I think the mustang will also be a much nicer car to drive. And since I'm not a frag racing guy, I think the chassis on the mustang will be more fun to throw around some turns. I look forward to hopefully getting some seat time in a '15+ GT at some point.