IMHO, those are both a bit dated. And the price is only *just* the going rate on the used scene. Not something like a steal.
The XLT has an annoyingly slow response time from the swing/pass , to when the VID appears on the screen. And being a faster swing 4-filter, is not going to be good for recovery speed, target separation, see-through-averaging, etc..... And the depth, when compared to other machines available today, has been surpassed. The XLT is 25-ish year. old technology (if you include its predecessor: the spectrum).
The 5900 Di pro there will give you about the same depth, but just minus a lot of the adjustable whistles and bells. The response time (from when you pass, to when the needle decides where to stop) is much faster, IMHO.
The 5900 is nothing more than a 6000Di pro, minus the auto-ground track. If you're in an area where you don't need auto-track, then this is a good machine (given its dated depth). But w/o auto-track, you wouldn't want that on the wet-salt beach, for instance (you'd have to be constantly re- balancing every time you stepped closer to, or further away from the water's edge). But this would be a decent machine for where you're land hunting on flat ground , or any such ground where the minerals stay "constant". The 5900 Di pro and 6000 di pro is also a good 25-ish yr. old technology.