Speaking of ppm's, I recently read that Tucson's tap water is around 450ppm and Phoenix's is 600+ on average... Is that true? Is RO the only way to go in AZ?
This is very complex; up until 2001, Tucson got all of its water from ~200 wells. Around 2001, they added in CAP water (canal water), which served a couple of purposes. The first thing is that it allowed them to shut down some wells that were already drawing down too much- after all, recharge isn't very much here in the desert, as it doesn't rain. The second thing (I think- I'm not sure) is that they were able to shut down some wells that had too much arsenic in them. The feds lowered the allowable limit of arsenic from (I think it was) 50 parts per billion (ppb) to 10 ppb.
Now, here in the Valley, we have some wells that have too much arsenic, too; some wells have treatment systems to reduce the amount of arsenic coming out to below the federal limit of 10 ppb, and others blend water from different wells so the net concentration of arsenic satisfies the federal limit.
Anyway- Phoenix gets its water from wells, CAP canals, and SRP (Salt River). There's also "reclaimed" water, but you're not going to find that coming out of your tap. Tucson probably gets reclaimed water, too, for agriculture and irrigation. Less than 1% of your water bill goes towards water for direct consumption (drinking/cooking), but all the water plumbed into your home has to meet federal guidelines for consumption- including showering (which is good, because arsenic is also absorbed through the skin, and there are data to suggest a lifetime of showering in water at the old standard of up to 50 ppb arsenic may be bad for you; whether 10 ppb is "good enough" is under debate).
Now, anyone who gives you a flat figure like 450 ppm TDS or 600 ppm TDS- I'd have to ask how they came about that figure. If the Salt River is flowing well (like it is now, after the recent monsoon) and the water coming out of your tap is largely from SRP, then the TDS is bound to be quite low; middle of the summer or winter, during drought, and it's from wells- then it's going to be pretty high. This is also going to vary between cities in Phoenix Metro, as each has their own wells.
TDS can change based on seasons- and probably even day-to-day, depending upon which supplies are running best, and what laws have to be followed (water rights based on how much each state is allowed to drain out of the Colorado and into the CAP canals), and recharge (when it's cheapest/best to run water from the Salt or CAP into the ground, like the
Granite Reef.
With all this in mind, the best way to go is reverse osmosis; it will produce enough water to meet the needs of your plants while maintaining a consistent product- important for when you're going to harvest a valuable crop. However, one must bear in mind that you're stripping all the nutrients out of your water when you do this; locally, tap water has plenty of calcium and magnesium- which growers pay extra for in the form of CalMag and similar nutrients. However, other ions (chloride, sulfate, etc.) are abundant, and of less importance to plants, and may negatively affect growth. It is also very important to note that once you strip out the solutes, your buffering capacity is shot: you will get radical pH swings as there's no buffering capacity. Some growers may find it beneficial to add back in 10% tap water- for calcium, for magnesium, for improved buffering. At those levels, you would still have some amount of dissolved solids, and most people who had ~50-100 ppm TDS for supply water would be very happy at those levels.
Also note that RO discharges a lot of water as waste; it is a wasteful process. There are "zero waste" RO systems (the reject water is returned to plumbing so it can go down the toilet or the shower or whatever) which will reduce the amount of water that goes to waste.