Popups are great for young families. Being smaller, it would be easier to store and easier to pull. People move up to travel trailers for a reason. To me, the biggest negative to moving down would be that there will be a lot less room to put gear/supplies. As the kids grow, you would miss the extra storage space even more. When we had a popup we kept a cooler in our TV since the refrigerator was so small. How does the refrigerator in your TT compare? If you will need to start bringing a cooler along, then you will lose space in your SUV.
If you decide to go for the popup, check that the lift mechanism works smoothly. I’d also look the canvas over closely. Good luck!