Even though I only have the 2720, I have a friend that has the 2720sl, and that puppy is sweet.....maybe 1 of the 2 models mentioned here in the future for me.....I hope, I hope.
Deciding between the 2 would be difficult. You would have to decide if the king bed (where you can sleep with your feet towards the hall, preventing you from having to crawl across your bedmate) in the 3124ks, is more beneficial than the extra 320 lbs. of tow weight in that model over the 2720sl. A lot would depend on your tow vehicle towing capicities. With a 5000 lb tow limit, a fully loaded down (with AC, awning, cabinets, camping gear etc.) 3124ks might be getting close to that figure. If you have a larger tow rating on your vehicle, then I would think it would less of an issue. And the extra 4' of tow length might tax the ability of the 4 Runner in a panic situation.