Hello P @ F
There are more advantages with using resin only than just less water usage. For example, the speed that you can produce pure water. Its not unusual for a resin only system to completely fill a 1,000 litre tank in around 25 minutes. (It will literally fill as fast as the water will come out of the tap)
Also, a resin only system is much simpler than RO, and therefore there is less to go wrong.
AND don't forget, that you can fill a resin system anywhere, for example from a customer's tap. If you're on-site and run out of water its not a big problem.
The costs seem similar (once you've taken into account the waste water cost with RO). Don't forget, using RO costs money as well - there are pre-filters that need replacing periodically and also the membranes themselves don't last forever.
(It's possible to fiddle with the waste water output of all RO units, not just the merlin, but this greatly reduces the membrane life.)
With tap water of only 100, which is pretty soft, I'd probably just go for a DI system. Much less hassle, and the speed of production would swing it for me.
-Philip