I saw this post too late, but the system with the water softener is what I would have recommended too. The water here in central TX is very hard too, and most people have a whole-house water softener installed. We do as well, but I generally don't bother hooking up my hose to it.
A couple of tricks: Never wash your car in the sun. The water evaporates so fast that water sports are almost impossible to avoid. I wash mine very early in the morning, or as the sun is about to set.
Now as far as some of the other systems are concerned - deionized water (DI) is ideal, but you can't recharge the filters yourself (unless you want to mess with concentrated hydrochloric acid and caustic lye). That means that you'll constantly be buying replacement filters, and those aren't cheap. They may not last a long time either, depending on how many ions need to be removed. I'm pretty sure that the Mister Clean product is basically a cheapo DI system. With DI and an integrated filter, you could theoretically let your car air dry, as there's nothing left in the water to deposit itself on your car's paint once the water evaporates.
The water softener you ordered should do the trick. It's basically an ion exchanger. The resin bed is charged with sodium ions, and when hard water with lots of dissolved magnesium and/or calcium flows through the resin bed, the sodium ions release and attracts the hard minerals. After a while, the resin bed becomes saturated and needs to be recharged. This happens automatically with our whole-house unit which is filled with several hundred pounds of salt, and it looks like your little filter unit can be done manually. The concentrated brine solution "re-activates" the resin bed for use. How long between recharging depends on how hard your water is, and how much you use.
There - all you ever wanted to know about water softeners.
