Kinda depends, IMHO. I've got two - 163" Summit XM and a newer Skandic SWT utility.
The SWT is by far and away the most comfortable for moving two (or three or four) people uphill.
It won't break a trail up steep soft snow like the Summit will.
The Summit will take two people up just about anything. It is kinda ridiculous what it'll do.
If I had to choose one, though, I'd choose the Skandic - it is more useful more of the time, and with a taller track (1.75" lugs vs stock 1.25"), it breaks trail up MOST things we want.
Lynx (where the Skandic actually comes from) makes a sled called the Xtrim Commander 800 - basically, a 20" wide sled with a-arms and 800cc etec. I've not ridden one, but I'd venture to guess that'd be the ultimate sled-skiing tool - the 600cc etec SWT is really pretty capable; give it an 800? Huh.
It really depends, though, on where you are, if you've got roads in place, if you need to make roads, blah blah blah.
I'm pretty sure *my* XM (2013) is capable of tandeming up stuff I have ZERO interest in tandeming up. I'm SURE the new T3 with a 174" track will double up things that are downright terrifying.
My Skandic is not as capable as a mountain sled WRT breaking a trail/making a road up steep/deep slopes, but once a road is in, the Skandic will go right up - and with an additional 30-40hp, I suspect the delta between what it CAN do and what I _want_ to do will get even smaller.
So...yeah. Realistically speaking, cost no object, 174" Summit.
Going all-in on sled skiing? Modern Skandics are awesome.
Spending $5-10k? Probably any of them, really. Longer tracks help. Power helps. I like Skidoo, but I've tandemed on new Pros and a couple of Cats - they all worked just fine. Heck, if you really don't care about the sledding piece, a Yamaha would probably do the trick if there are established roads in place, one of their mountain sleds with a big track.
Where do you plan on using it? Making your own roads or using established trails?