I've always told newbies that the easier you make maintenance, the more likely it is to get done.Like a couple others have mentioned, my RODI goes into 32 gallon Brute and I just mix salt in separate tub the night before use.
*all extra RODI and saltwater I store in capped 5 gallon Sparkletts jugs and have used both weeks or even a month after storage without any noticeable problems.
I will say this, I really am starting to wish that I had just setup a mixing bin/station also when I did my RODI setup so, if I had to vote, I'd vote BOTH (to the original question)
When I do my weekly WC's(Just old school, and have always done weekly WC's), I just flip a valve under my tank, push a button(hooked up to an apex that turns on a pump that pushes out the old saltwater to a drain line I installed, shuts off the return to drain down, turns off my skimmer, and reactor). I then just turn another valve to pump water back into my tank.
I could do it automatically with my extra DOS, but I like having to do it manually, as I clean other things during this time, and it keeps me "in tune" with my tank.