I'm seriously considering acquiring an SLK as my second car, but it appears I have some delusions that must be solved.

Is the 5-speed SLK 230 any faster or slower than the 6-speed? Also, is there a noticeable difference between the 230 and 320 engine?

I have driven Both a few times. I honestly can't see too much of a perk in the 320. I'd get a AMG sport package 230. The sport package makes it look like a totally different car. I was super disapointed in the 320, 30 hp makes almost no difference in the acceleration. The sound on the 320 is much much better than the 230, but you can always mod the sound on the 230. Youll know when you find the right SLK.

In terms of real world fuel consumption, both engine are about the same, maintainece is slightly higher on the V6 (12 spark plugs vs 4), real world power is similiar due to the shorter diff ratio on the 230. In terms of feel and sound the V6 has a big advantage, it sounds more "relaxed" and doesn't sound like it's working hard.

As for transmissions, stick with the Auto (5speed). the manual is ok but it's not really an particually "involving" transmission. You will get bored of the manual and clutch feel doesn't suit performance driving.