Both are fine bikes. Don't worry about the 650 cvt too much. There's too much hype about the belt longevity. Yes it will eventually go out and yes it is expensive to fix.
But by most reports you'll get 75k out of it. In that amount of time you'd have three belt and roller changes on the 400 and at least one, dry clutch replacement. So tranny maintenance cost is about the same on both bikes for that amount of mileage.
Other than that, there's few other reported inherent issues with either bike. They're rock solid mechanically. Just rubber, oil, filters and such every now and again.
The 650 does have a tendency of chewing through rear tires at about a 5-7k mile clip. But that's true of many larger bikes. Can't fault the bike for it,
Base your decision on what type of riding you do. Highways and long trips, 650. City commuting with occasional trips, 400. Both bikes will do any type of paved road riding, but they each excel at their own niche.
Good luck, apprise us of your journey.