There are two families, if you will, of Urals, and they're actually quite different.
The two wheel drive family (which actually includes a one wheel drive model, the CT) is built more for primitive or off road travel. The bike and sidecar chassis are higher for greater ground clearance, and the hack wheel is in line with the motorcycle's rear wheel because the driveshaft to the hack comes directly off the rear axle. It also has an Earle's style front fork, different gas tank, and other differences including a stiffer, more off-road ride. Because of the higher c.g. and geometry of the 3 wheels it is less inherently stable than the M70, and thus perhaps should be driven with more caution at higher speeds. (Higher speeds for a Ural, of course

)
The M70 chassis is the one that copied the old BMW, and it's sits lower, wider, and the hack wheel is forward of the motorcycle rear wheel, which results in an inherently more stable triangle. It has standard front forks, a different gas tank, and other differences, and the result with the lower cg, wider track, hack wheel position is a more stable, softer ride more oriented towards the highway.
The user manual states that the top speed is 70mph. It will do that and the rig still feels stable, but it does feel like you're flogging the engine way too hard. If I'm in a hurry (which you shouldn't be on a Ural) I'll do 65, but we run much more comfortably at 55-60, which it will do all day. Yes, it's a backroads bike, but occasionally we have no choice but to get on a freeway, where we sit in the slow lane at 60 until we can get off.
So, the salesman is pretty much right from the standpoint of the M70 being better on the highway than the two wheel drive and CT models. That said, IMO the bike's engine starts getting abused to go faster than 60, and the manual itself says 70 max.
There is a LONG thread at ADVrider called "Why You Shouldn't Buy a Ural" which, in fact, helped convince us to buy a Ural.
I didn't mean to turn this into a Ural thread, and would refer anyone to Soviet Steeds for tons of info, and I'll close by saying we've had ours for a year, have had one part replaced under warranty with no fuss or cost, and have been very happy with it.