Either unit will be just fine. It is more of a Ford vs. Chevy debate.
The c100E resin is fine, Res-up feeder would not hurt, nor is it really needed for that low of iron. 50/50 on that. I would simply add some resin cleaner to the brine tank every 6 months.
18x40 is nice, you will fill it less often, but it is also taller. If you have difficulty lifting 50 pound bags, the shorter tank may be desirable. I prefer the larger tank. Fill it every 6 months and ignore.
Vortech tank, neat idea but not worth the extra $