As far as stability they're pretty good. I have had one set up outside in the past (didn't have a yard) in an octagon shape, and it tolerated older puppies/young adults jumping and banging around in it. I even had one puppy who would jump up and "hang" from the top with his armpits over the edge while he watched me work outside. lol And he was a big boy. He knew how to climb out easily, but would only do so if I left him alone outside for more than a few minutes or if I was walking away and looked like I was going to go somewhere special and have a lot of fun. lol He just liked to hang there though a lot and remind me that he DID know how to get out. And yes, all my xpens are 42".
The tops add a significant amount of stability, but limit your shape choice to only a square. I like the midwest top because it has wire that comes down and around the top edge of the pen, unlike some other tops that are just a flat grid that hooks on. The Midwest top has about a 1" lip around all the sides that help keep in on nicely. I'm not sure if any others have the lid like that? It looks like the JB one does.
Regarding the size, if you're worried about cramming it in a car, I wouldn't go any bigger than the 42. I used to have a T-Bird when I still had 2 dogs and could fit the pens in my trunk just fine. However the tops are 48" and those would not have fit. I had to pin tarps across the top until I got a van. You could certainly get by with a 36 if you're worried about portability, as long as you don't have escape artists. With a 36 they can get their feet on the top row while still standing on the ground. But I know of a lot of folks that use a 36 for Basenjis. Also keep in mind the taller they are, the HEAVIER they get to lug around! lol