I have the RDA/RTA challenge. The flavor is better on an RDA, but I don't think they are really portable, whereas the RTA excels at that.
I have both and the RDA are for sitting at home. The RTAs, I use, are a mix of full size and mini/nano. Yes, I do have a few. I seem to get vapor's tongue rather quickly, so I am switching tanks pretty often. To keep them all straight, I made the following so that the bottle sits in the stand just behind the topper. The top row is for up to 15ml bottles and the bottom is for 15ml Glass up to 30ml.
Hope helps