Well, I'll get the ball rolling here ...
Inseam?
Strength, particularly regarding ability to hold up the bike?
Intended use?
Which is to say:
- The 650 is a heavy bike, with a tall seat. (Not as tall as the Beemer 650s, for instance, but still tall.) At 600-something pounds, it can be a handful at stops, and getting off the centerstand. I don't know the details on the 400, but it's considerably lighter.
I went from a 650, owning it for five years, to another very heavy bike, a Victory bagger. The Vic was actually almost 300 lbs. heavier, but I could flat-feet the Vic MUCH more than I could the Big Burger, and so felt much more comfortable having my wife out back on that big bike. (And I went back to another 650 Exec, and sold the Vic, after my wife had medical issues that stopped her riding.)
- Do you plan on doing short-ish trips, touring, or both? And possibly two-up? The 650 is the better tourer, and can cruise all day with the fastest superslab traffic. If that sort of riding will be a rare occurrence for you, then it doesn't much matter.
- The 400 is probably easier to work on, should something major go wrong. I'm thinking about the respective trannies; if the one on the 650 goes, you have extensive work ahead of you, or you can convert it into a mailbox.
- The MSRP of the 650 has always been a knock against it by those not in the know. For those of us in the know, its wind protection, storage, handling, and just all-around fun negate that. All of this applies to a lesser extent regarding the 400.
If you decide on the 650, perhaps a dealer with an aging, now sort of obsolete, floor model will give you a good deal on one, because it's not making the dealership any money -- and likely costing it some, regarding the money tied up in it -- just sitting there on the floor. Lots of the for-sale ads are from folks who wish they could get whatever they're asking for one, but that doesn't mean they'll actually get their asking price, or that they won't accept a lower price.
Glad to see that you want to get back in the saddle. Best of luck with your search.