I think the SLK roof is better as having a soft and hardtop.
We have several SL's and the biggest problem is getting rid of the hardtops somewhere without a possible chance to damage them.
The soft top suffers a lot when it is packed in it's trunk; the cheap plastic rear screen gets worn very quick when the car is driven a lot with the hardtop on.
Larry,
I do not know what model SL you own, but in europe we have OEM rear seats for the W113, W107 and R129.
Mercedes had a 1-person rear seat for the W113 and a 2-person rear seat for the W107 and R129.
I have a rear seat (with seat belts) in my 1990 300SL-24 and can send you some pictures if you want to see some details...
The rear seats cost about $2300,- and will only allow you to put in two (max 8 year old) kids without legs...
The back of the SL is just too small to put anyone in there and I always have a lot of trouble putting in a third grown up in the back of my car.
...let me know if you want to receive some pictures...
greetingz,