I own an N24 and I think its fantastic grows chaeto like crazy I do dose iron/mang etc. With that said it is on a much smaller system than yours but on my new build I am switching to CW ATS as honestly it is quiet large and the process to clean out some chaeto is easy enough but requires space and a bit messy, but if you have a fish room should be easy enough.
As for sizing I think you would be good as I am assuming you are using fleece/socks and skimmer. The one difference to go bigger is you can go longer without having to trim it which is why I went with 24 opposed to 18 on my system.