I liked the idea of All-for-Reef for Nano tanks since the cost is low (low volume + powder version) and it's convenient. With two part, you would be missing the added trace elements and potentially deal with dosing pump accuracy - when I used to dose two part, I would notice that one container will drop faster than the other (even though I'm dosing equal parts), evidence of dosing pump accuracy issues.
With A4R, however, you would have to deal with a delay effect and wait a few days after adjusting before you'll see a result. So it's a potentially much longer period of adjustments.