The more prefilters the less often you need to replace any given one, and the more protection for the all important membrane. For example if you have an extra stage you can run a 5 micron carbon, and then a 1 micron carbon which allows you to prefliter down to 1 mic without having to constantly replace that 1 mic prefilter. Without that extra stage, you need to either go with a 5 micron and send that lower qulaity water to the membrane, or go with a 1 micron but that will need near constant replacing. In the end the operating cost will be lower if you have an additional stage or two.
On membranes, you want to be sure to get a quality membrane with a high rejection rate. For example filmtec is basically the industry standard, but note that their 75 gpd units have a notably higher rejection rate than some of the others such as their 100 GPD membrane. If you get a lower rejection rate, you will blow through DI cartridges far faster than with a very high rejection rate. Either way, the membranes cost about the same and last a long time. DI cartridges get expensive fast if you need to change them a lot.
Also since I don't think anyone said it yet, RODI systems are basically all the same, or rather are all made of interchangeable components. Point being, If you compare units with the same specs and the same bells and whistles they generally can be compared equally.
I'd suggest looking for at least 4 stages with a pressure gauge, Auto shut off, membrane flushing bypass, and dual TDS meter. You can save a few $ up front if you get some thing without all of those things, but in the end you'll probably regret if you do.
IME the AWI typhoon III is a great unit for the price, but I suspect that you could get something with all the same features for competitive prices from bulkreef supply or some other vendors. Also IME AWI customer service is excellent.
