Well, the first thing to note when deciding between the two is the price points. The Velocity is somewhere near $125 and the mutation x v4 is $30. But after reading up on the Velocity, it seems the price is worth it, as the machinists hand check every part for imperfections before the RDA is assembled, and is USA made, where the Mutation X v4 is Chinese made.
I have the v4, it's a great atty for the price. My cons with it are the thinness of the barrel section and the AFC on the top cap. And the plastic spacer ring thing makes it look cheaper than it is when it's put on. Also, when you close off the side AFC fully, only using the bottom air flow, air still comes in through the side holes, as there is a tiny gap between the top cap's AFC and the barrel, letting some air in.
The Velocity looks like a solid investment to me, you have the options of two different air flow sections (same with the Mutation), but with a two post design. I love two post atty's, building and centering coils on them is super easy. The 6 circle primary air holes look like they provide a good amount of circulation before getting to the secondary cyclops air flow.
I would get a clone of the velocity before buying authentic, as the price difference between the two authentics is pretty steep. Clones can be had for around $10 online