Function as has been recommended. Nothing worse than what lessons cost. Buy small, need bigger, We also went from a truck camper to a 35', but gas. Nice size for us. If I had the chance to get a 45' without the cost of that lesson, not even sure I would take it. A 40' is nice, and National Parks typically have room for you. Not all CG's, but most.
Again, function is the key. If weekends then smaller will do for a long time, you adapt. On the road fulltime, go for the layout.
One thing though, you almost need the wrong ones at first, just so you know what the right one is.