My wife and I have ridden both, and own neither. We seriously considered buying the SX for her, but I liked the USX better. You didn't say how tall your dad is, but if he's 6-0 or taller, he may have trouble boarding and riding the SX. My knees hit the handle bars on the SX if they were low enough to be comfortable for me. I really lliked the USX better because of this. My wife on the other hand didn't like the USX for the same reasons. She is quite short, barely 5-0 and had trouble boarding the USX. By the time I'd slid the seat forward enough for her to reach the pedals, the handle bars were too far back to be comfortable.

I'd really suggest that before you spend the money, take him to the shop and let him try them out. Maybe they'll let you keep one over the weekend. That way, you can get the one he likes best.

Oh, the reason we didn't buy a trike? We live and ride on dirt roads, and they are both built a little close to the ground. The rear derailer would sometimes hit a rock or the ground if the wheels were in a hole or rut. And if you do get him an SX, tell him to watch out clipping a curb. My wife nearly toppled over a couple of times by cutting a corner a bit short and hitting the curb with the rear wheel. I didn't have that trouble with the USX but I don't really recall hitting anything either. On the other hand, it didn't take much to right it, she just leaned the other way real hard and it sat back down.

On the other hand, she loved the ride once she got used to its little quirks. And I'd love to have a USX. They are a blast to ride on paved roads.

Good luck with it. Either machine will serve him well. Just pick the one that fits him best.