If it's just going to be a garage queen it might not matter much, but if he actually plans to drive the car, the Challenger would be the smarter option. The Viper has a permanent spot in the annual top 5 most expensive cars to insure list. It has finished #1 (or last depending on perspective) on Forbes most expensive cars to repair list, and it also has a G.I. Joe kung fu grip strong grip on top 5 (often #1) finishes in Consumer Reports most expensive cars to own list. I don't normally pay much attention to such surveys, but the Viper does so piss poor in almost all of them that it necessary to point out. The up front cost may be similar, but when you compare operating costs and additionally factor in that the Challenger will have a warranty, and the Viper won't, financially speaking, a Viper is not a viable option in this price range.
It's really not that bad. And of course I was being slightly facetious, but if someone really wants a fun car some slightly older, low miles cars can be surprisingly good options.
For insurance: Full coverage on my RT/10 is $15 more per month than my Mustang GT (2001 Viper, 2002 Mustang GT). That's ~$80 a month vs ~$65. I've pretty much maxed out Progressive's options, $500K CSL with $100 comp/$500 Coll deductibles. I'm 29, clean record.
Tires and maintenance will obviously be a lot more ($500 rear tires...), but it's likely more than offset by the difference in depreciation curves. [EDIT]Just looked up tires for the Challenger, $1240 for the OEM tires vs $1870 I just paid for PS2's. So 50% more on tires.[/EDIT]
When you consider the full picture I would make the argument that barring catastrophic engine failure or some other equally devastating mechanical loss the Viper may well be less expensive to own.
Whether it's a Viper or something else the point still stands: Take advantage of the flatter depreciation curve on an older, far higher performance vehicle and do the math - It might surprise you.
Viper GTS