Hello godzilla. I have used and owned both units. Both are excellent choices, however the discrimination characteristics of the Excal clearly make it the better choice, imo. On the Mark II, there is a feature called Discrete Trash Elimination, but that is not the preferred mode of operation. Standard Trash Elimination, 0 on elimination/discrimination and barely audible volume/threshold is the way to run the Mark II. I would HIGHLY recommend you spend the extra $ on the Excal. I have personnally had greater success using that unit, over the Mark II and they are probably comparable in depth.
Buying a used water unit is tricky business. You just have to be careful about what you get. If you buy one from someone reputable, like say myself, then all should be fine though. I have one for sale on the Classifieds now. My equipment is in tip-top shape, as I use it all myself and I don't use junk out there. I used to own 3 Mark IIs, now I own 3 Excals. In the event you DO buy used, then a trip to factory is not a bad idea, to ensure the unit is operating correctly. Mine just came back from Minelab too, so all is well. Best of luck on your decision mate.