i would recommend the 7 speed auto. the manual is good, but i felt there was a gear lacking! the extra 7th gives alot more freedom in the gear ratio. lpus in traffic its such a breeze without a clutch and comfort on. the paddles are an absolute must with the auto, as you could get frustated without the option to drop a gear a 2 when needed.
The 6-speed manual is a dream. It makes the car so much more engaging and fun. So much so, it's worth not having the 55 engine just to have. I would only go with the automatic if you are in traffic such as LA, Manhatten or Calcutta. Even in Miami's traffic, the smooth 6-speed makes you smile.
Oh yeah, and with the 55 engine. The gearing of that 7 speed seems much better. Each 350 I drove with an auto seemed to be ALWAYS in the wrong gear, and sluggish to adapt. These were with new cars. Later on, when I drove a C Class with the 7speed, and it had 15,000 miles on it, then that transmission actually felt right.
So, if you can't try a manual (as is impossible here as they only stock automatics) and try the automatic, know the stick is absolutly wonderful. Also know the automatic improves with miles, so your first drives might not be accurate to how the trans adapts.
One more thing: If you care about what people say (and I do not) know I've had several people say, "oh an SLK with a stick, so not a chick car."