Tony,
I was going to reply to your post last night, but I was tired and figured I'd get to it in the morning. This is how it's going to go down with the SRT Coupe and the Z06: the Z will be faster in the 0-60 race then the Viper because of the light-weight, shorter gearing, and higher rev-limit. It will also be a pretty damn good track car, like the Viper already is. We know GM is gunning for the Viper and I'm sure once the test results come in from the magazines, the numbers on the Z will be awesome. Enter the SRT Coupe: there are rumors floating around that the Coupe will be introduced with 600 hp at the crank; if true, PVO played their cards right as far as the SRT vs. Z06 program is concerned. If untrue, the Viper will still be a much different driving experience then the Z will and that will boil down to the characteristic which is most often overlooked by bench-racers: torque. The SRT produces its 500 ft/lb's at damn near idle and maintains this curve throughout the rev-band. This makes the car feel and respond much differently than anything with less tq more hp; ala 'Z'. The Vette will probably be more comfortable, have more gadgetry, and will be more civilized. The Viper is a much more raw driving experience, will have much more 'curb-appeal', and will still have the exclusivity going for it.
Looks are entirely subjective; I think the new Z looks great, but side-by-side, I'd still take an SRT coupe. Another thing to think about is the reliability and longevity of the new Z motor - GM has never made available this large of a displacement motor at the factory level. Combine this with the high-revs and the brand new dry-sump system and there may be cause for concern with the first year of the production run. Not to say the SRT didn't have problems, but the basic architecture and execution of the Viper motor has remained consistent for 14 years now and I think that says something about it's hardiness.
Also, think about how you will be using the car - are you a polisher or a driver? Will you be carting the car to the track or will you cruise on the streets? If you're a track-rat or would like to be one, there is a specific racing series for the Viper called ViperDays and is one of the most comprehensive and kickass things you can do as an owner/driver without breaking the law. Also, the Viper community is tightly knit and has a ton of support and if you take your Viper to a large, reputable dealership, you will have your car serviced by a certified Viper technician instead of some GM Goodwrench Service Plus dude.
On a side note, this is pure speculation on my part - no one has driven either car or flogged one for an extended period on the track. On paper, it looks as though GM has finally caught up to the Viper in terms of performance and track-readiness. As with any car, the first run of cars built are bound to have teething problems, but IMO and since I'm a Viper guy, I'd stick with proven performance, the history, and the legacy of the Viper over the new Z, but I think both cars are going to be exceptional and ground-breaking in terms of at-the-limit transitioning, braking, and acceleration.
Good luck on your purchase!