I actually rented the Challenger and the Camaro back to back. The Camaro is faster and more nimble, but it's ugly as sin (personal opinion) and has the worst visibility of any car I've ever been in. The Challenger is more form than function, with really classic styling, but it's much more comfortable and spacious. I wouldn't even bother with the Camaro, if you own anything relatively sporty at home I think it'll be a disappointment (IE: I drive an M3 and found the Camaro to be pretty lacking in handling/fun as well as comfort).
The Challenger would be my pick. It's not exactly a rocket ship, nor does it handle like a Porsche, but it's got a feeling of classic Americana which I think would make the drive from LA to Vegas and cruises on the Vegas strip more fun.