I think either of them would do the job. Mechanically I'd say that all the three bikes will endure the trip without too much trouble.
One thing that springs to mind though, is the fuel capacity. In my oppinion, both BMW's have too low fuel capacity to be regarded as traveling bikes. They both have a 16l tank which is simply too little for a traveling bike. It's fine for for the western world where filling stations are on every corner. But if you go into the more "rural" areas, you will need to figure out how to carry extra fuel. Jerry cans will only take you so far, besides being inconvenient to handle and strap to the bike (takes up space). I know that you can get an extra large tank for the 650 at touratech.com. But that costs 1600 Euros! Also I've been told that you need a engineering degree to install it. I met a guy who spent almost three months getting it to work properly. And then only through major modifications that weren't in the "kit" (I don't have details). So on top of the 1600 Euros, he spent quite a bit of money going back and forth to a mechanic, whom he also had to pay for his time. But if you're willing to spend the money and the time, then you get a 39l fuel capacity on your bike.
I am unaware of any extra large tanks being made for the 800. But if the price for the bike itself is an issue to you, then I don't think it's even worth looking into, since it is bound to raise the total cost even more.
On a sidenote let me say that even with the 22l fuel capacity of the XTZ, I recommend a 5-10l jerrycan on top of that.
There is also the issue of spares that you need to consider. You will need to buy catalyst free cans for your bike (depending on un-leaded petrol availability on your route). Do some research regarding how much a set of catalyst free cans will cost for the BMW's as opposed to the XTZ. Along with all the other spareparts you will need to buy for your trip. There might be a big difference in the cost there too.
I personally have the R with a 26l extra large fueltank from OTR fitted. As opposed to the trouble and price of the GS Dakar tank from touratech.com, I payed 900 Euros for mine and fitted it in less than an afternoon. And it worked from day one (bar some minor adjustments). I drove through Africa (Denmark to Capetown, western route) on it, without missing a step. Seeing as the engine and much of the "technology" on the XTZ is the same as on the R, chosing between the three bikes you mention, would be easy for me
. But it does you credit to do some research before jumping the gun on what might be THE most important investment of your trip.
But as I said in the beginning: EITHER of those bikes will get you through the trip. There is much to be said about chosing the bike you feel comfortable with. If bike "A" just gives you a good feel and puts your mind at rest, then go for it. The money you spend on "peace of mind" can never be regarded as expensive.
Best regards
Torsten