What is the best solar water heating system?

With the amount of sun you would get in New Mexico, direct solar thermal collectors make a lot sense, though in your case there is also an argument that can be made for powering a standard water heater (or heat pump water heater for that matter) with your PV panels. That way when there is no requirement for water heating, your system will still produce, as any power collected can either charge batteries or be fed back into the grid for credit. Since you already have a Solar PV system and the infrastructure in place, I think that is certainly worth looking into.  

If you do go that route, you could also consider getting a water heater that runs off DC, that way there would be no loss from the inverter. 

If you do decide on a thermal solar collector in the end, we do like that Vitosol Solar Thermal Panel from Viessmann. Overheating is the main cause of system failures, and they have developed an excellent solution to that problem.  

As for the SANCO2 Heat pump water heater, that is a fantastic piece of equipment, we looked into it a while ago but the price was still a bit prohibitive in our opinion, however, that may just have been Canadian pricing, and it was over a year ago so maybe it has come down. I've contacted the company to see if they offer it with DC, nothing back yet but I will update this when I find out.