Not owning an SRT, my observations come strictly from what I've read in magazine road tests, and here on the forum.
Essentially it basically comes down to aesthetics and tires. Of course I'm comparing factory stock SRT's vs stock Supercharged SLK's.
Preformance (acceleration, handling, and braking) should all be marginally better in a Crossfire, simply because of it's tire size, or "foot print", as opposed to the narrow, and taller aspect ratio tires used on the SLK.
On the other hand, the Mercedes should offer a more comfortable, and compliant ride.
Ironically, (when comparing the NA version of both cars) it's those very same narrow tires on the SLK that makes it accellerate faster than the Crossfire by nearly a 1/2 sec. in the 1/4 mile. Due to less reciprocating weight.
Reliability should be a dead heat, but in the SLK's favor, they are allowed into a Merc dealership, where it's usually "members only" when a Crossfire comes a knockin.
After all is said and done, it simply should be which one looks the best to ya.
Oh and there is that dissappearing hard top thingy on the SLK you have to deal with as well. Some say it's a plus, but I think it boils down to how follicly challenged one is.