The Mustang comes with no stereo, no ac ... ect. I'd personally rather be sporting a Viper then a mustang that costs $50K. I don't imagine most people are going to be pushing either car to it's limits ... and the performance numbers are close ... so get what you want. I gaurantee that the Viper will get more attention though.
If I was going to spend $50K on a mustang I'd just get a cheap one and put on the parts I wanted.