Are you talking all authentics? The legion is great to build on with the way the posts are set up, and the center post hole is huge. It's actually the first RDA I bought when I got into drippers because it looks awesome. It's got a massive juice well and you can fit pretty big coils in there if you're into building and such.
The mutation x v4 is pretty standard, I personally would pick something else before it, perhaps a STRO v1, it's the same price, $29.99, and it has the deck of a competition Kennedy with side AFC similar to a Royal Hunter. It basically looks like a combination of the two, including a honeycomb in the wide bore drip tip. The mutation x v4 feels flimsy in a way, the chamber is super thin metal.
I can't say much on the plume veil as I don't have one, but I know it has multiple airflow options, including air that comes out through the negative leads.
Are you looking for flavor, clouds, solid build deck to fit fused claptons and other large coils in the posts? What's most important to you out of an RDA?