When we were looking we started looking at the 20, 23D and 23FB. We thought we could get away pulling one of those with our 4.7L V8 4Runner. We had a 17' Casita before at 3500lbs. We didn't like the corner bed/bath of the 23D and the small gray holding tank (smaller than the Casita. We ended up with a trading in on a preowned Tundra 5.7 and a 25fb. We looked at a 25SS a few weeks ago and even though wider than a 23 the rear bed/bath still didn't work for us.
We looked at a 20 and liked the floor plan but the preowned unit we look at was in bad shape. The 48" bed was smaller than the Casita's 54" bed and the grey holding tank way smaller, 21 vs 32. The 20 only had a 4 cu/ft fridge, same as the Casita. If we hadn't owned the Casita we would've kept looking for a 20 probably.
Having the 25fb I'm not entirely sold on the rear dinette side couch living area. I'd prefer a separate couch and dinette but I don't want to tow anything bigger. Of the smaller floor planes the 23fb would be my 2nd choice now. It has the same bed layout as the 25fb, more closet space than the 25fb and much spacious bath but you loose the bath medicine cabinet in the 23fb and you loose the 2 door fridge. The 25fb has a 7 cu/ft to the 23fb 5 cu/ft with internal freezer. I'd prefer the 23fb in International trim because the kitchen has a round sink which gives more counter space. The 25fb has 3 exposed kitchen drawers but the 23fb has a couple of drawers behind the cabinet door which I don't like. The 23fb has a decent size gray tank, 30 gallons while the 25fb has 37 gal. The overhead AC on the 23fb would cool the 23 better than the 25fb but we've found it to cool our 25fb adequately even in the hot Texas summers. We have the full awning package so the rear windows around the dinette/couch are shaded.
Read this blog of a couple with a child that went from a 19' to a 25ss.
Adventures of Dave and Ann
They did well with the 19' but with a 2nd child now they found a preowned International 25ss. They ended up changing their tow vehicle for the larger trailer too (Toyota Sequoia 5.7)