The supercharger will account for some of the weight. Hard to be sure if that will account for all of the difference, but you should not look at it as just an R/T that magically added 100+HP. I'm not sure if they're adding an intercooler or not, which would be more weight, further forward. The SRT has a sportier suspension from the factory, although the Saleen will receive "Chassis, wheel and interior upgrades" as well. (Autoweek 5/26/08 page 8). Autoweek also reports that the SMS will start in the mid to high 50,000's range, so at least 10-15K over an SRT-8.

Handling is an open question at this point. Early reviews of the SRT are very positive about the handling, so I would not fear that the SRT will be lacking in handling. Tuner specials like the Saleen often push the boundaries of street car suspension tuning with lower ride heights, stiffer springs, bigger anti-roll bars, higher damping rates etc. So the SMS *could* handle better. But with no driving impressions, and no cars actually built, it's all just guesswork.

As purnrg implied, reliability is more a question with a tuner car with forced induction than on a stock motor, although Saleen has an excellent reputation.

I do not think you will make a terrible mistake either way, but I would wait to read some driving impressions on the SMS before committing to that option. Maybe a magazine will even do a comparison test and make life easier for you.