This question has arisen many times before.
You may wish to review
this post .
FWIW, i've driven both, and the 5 litre GT Mustang. The post above contains links to reviews I published elsewhere.
All three cars tend to be the same price. They are only in different categories so that if you book one, you should get the one you booked, not a random lottery on which car they actually have available. Hertz generally have good stocks of all three models in their fleet.
If it were me, I'd pick the Camaro SS, in Convertible guise if I was somewhere with decent weather... but the Challenger is a good drive and does have more space in it for people and luggage (though you sound like this is not a decision criteria for you).
Have fun!