The XT should have the subframe strengthened to carry luggage. Especially important if you want to take you and your luggage on poor surfaces.
Either of the XTs you listed were at the top of my list of "cheaper" bikes, along with the Honda XL. The semi-cheap would have been the XTZ Tenere, Honda Dominator, Transalp or Africa Twin. The expensive would have been the BMW 650 Dakar. And if I was loaded, the 1200 BMW Adventure.
As you know how to work on these bikes, I think I would have opted for one of the XTs. Further, as you plan mostly to ride on pavement, I'd go with the 500 (presupposing that the engine, clutch, sprockets, etc, are in good nick). Do expect to sink some money into whatever you choose in modifications, servicing, etc.
For my trans Africa trip I was only moments away from purchasing the Dakar, but ended up going for a classic Vespa px200e scooter at the last minute. These are anything but reliable and have poor performance along just about every attribute. I will bring LOTS of spares and tools, and expect to do a bit of wrenching by the side of the road. After I have scavenged all the parts and done all the modifications I need to do, money wise, I could have gone for the Dakar without making much difference to my wallet...
It is important to remember that there is allways a solution to any mechanical issue you may encounter anywhere in the world. How reliable you should go depends on how much you are willing to risk spending on the following: money, time and energy. Your bike can be put on the back of a flat bed and shipped to the nearest competent mechanic a 1000 km away, and parts can be couriered to just about any place on earth. Remember also that costs are inversely proportional to time. The faster you need something resolved, the more expensive it gets. This also holds true with anyone who is there to make money off of you, the more screwed they can smell you are, the higher the price. If you just
along on your trip, spending weeks or months in some places, I wouldn't worry much about anything. Being stranded in some willage while waiting for parts may be just what the doctor prescribed... you might even get to know somebody. If you however have a strict itinerary and time frame to hold to, then you need to be self reliant. This means opting for reliable solutions and/or having the means to fix any propblem quickly (access to tools, knowledge and parts). On my trip I am bringing parts worth about 1000 Euro... I have a strict itinerary and time frame, but willing to risk spending the energy if something goes wrong.