Jake,
You won't notice any difference in usage if you are not an experienced sprayer . There is a slight extra materials (ie paint or finish) saving with HVLP but it only matters where you are spraying huge amounts and the cost becomes noticeable.
The RP technology meets all current VOC legislation and so is "environmentally friendly" It uses about half the air of HVLP. This page
http://www.sata.com/Produkte/produkt.js ... ischedaten
has details and if you dig around the site you can compare all the various Sata offerings. My gun is the Sata digital. I ended up getting it in preference to a deVilbiss because the cognoscenti seemed to rate Sata guns as the very best and because it offered the advantages of HVLP without the high air consumption.
I haven't followed what every other mfr is doing since I got my gun but the industry now seems to be heading for a LPLV system (Low pressure AND low volume) I guess the RP is somewhere down that path - when I got it about 18 months ago - maybe two years, it was a technology leader.
I would emphasise that you can get good results with cheaper equipment but don't go for the very bottom of the market, that stuff is awful.