Ok, I'm fairly new on here, but coming from a saltwater background, I have a spectrapure RO/DI unit. I was looking at the PWC units at first also, and did a lot of asking around on the salt forums...
Basically what it's going to boil down to is efficiency. That will get you to 0 TDS, but it's HOW it does it that matters. Your RO section is going to be the kicker really. A nice spectrapure system will usually get you a solid 98-99% efficiency (rejection) rating out of RO. That means that if your water starts at 375 TDS (like mine) the RO will get you down to about 4 TDS or less, then the DI is what gets you down to 0.
This means everything will last longer, the DI filter won't be working as hard basically... If your RO efficiency is down to... lets say 94% with a lesser quality unit, that puts the TDS at about 22 by the time it hits your DI resin. Doesn't seem like that much of a difference at first, but that's actually making the DI resin work 550% harder! That means replacing it more often.
Also, the cheaper units come with lower quality, more brittle fittings, so you'll find yourself replacing bits more often.
Hope that helps to understand the differences... You'll just be spending more in the long run replacing lower quality resins and membranes, along with low quality parts. That and customer service may not be what you hope for if you have a problem, but spectrapure has been doing it for a LONG time, and their service is great!