Some claim the clutching on the 900 isn't so off-trail slow speed friendly.
I was considering an 900 ACE sport, but the 1200 SE wasn't really that much more expensive when I was shopping last year. I got $1200 more in discount with the SE than the Sport, so that made the decision easier for me.
I put a few hundred miles on a Venture GT over the last couple of years, and found the 1200 Expy SE to be better riding and performing. The Venture may have handled a bit better with its wider front end, but its wasn't a significant difference to me. Off trail was an incredible difference. I spent a bunch of time pushing and pulling on the Venture. Not with the Expedition. The factory winch hasn't been tested yet. If I was riding the Venture, it sure would have been.
I'd expect the 900 Sport to be comparable off trail to my Expy SE and maybe a bit better handling on trail. I'd expect the ride to be a bit worse, but still likely better than the Venture.
If your Dad is a fan of the Yamaha triple, Cat really shook things up with their utility sleds this year.