Before you buy, I'd visit all 3 models in person to see what would work best for you, and take the kids. Lounge around inside to see what it will be like when you're done with your camping day and just want to chill (and get them to chill so you can get them to sleep
).
Personally, I think the 2720 "regular model" (with the higher, permanent beds on each end) is best for a family. For now, both kids could sleep in the rear queen while you relax in the front after they go to sleep. You will be comfortable in the front full bed because it's longer than the couch bed or the dinette bed in the other models (unless you're not that tall, DH is 6'3" so it makes a difference for him). You won't even have to put down the dinette if they are willing to sleep together in the rear bed.
The 2720 SL and the 2720 SD (I've been in both) only have two beds. Our kids used to share a bed when they were little, but it didn't last long.
At night, before bed, what we used to end up doing was: Kid 1 took up a space sprawled out on one side of the dinette, kid 2 did so on the other side of the dinette. If I wanted to sit down somewhere, they'd have to sit up or crawl up to the bed at the front (or I did). DH usually sat in a fold-up chair in front of the door. We could all see the TV/DVD player from there. As soon as I could get them away from the dinette table, I'd put the table down and turn it into a bed.
But, if you're not thinking long-term (as in having teenagers staying with you) a different model might work. Before I knew it, after we bought our TM, I was the shortest person in the family. It's amazing how fast they grow.