I have a round column mill and I like the little critter for what it is.
It is tougher than the x3 because it is larger (I used to own one). And that means you can do larger work take heavier cuts etc.That is it's Primary advantage.
Round Column means every time you need more head room for a different tool say you want to use a drill chuck after you just milled out a profile with a 1/2" tool . You have to re-zero the machine or pick up an edge for reference. With the square column machine you can just raise and swap tools and you don't loose your place on the x/y.
It's a pain to set up for multiple cuts and have to reset everything. It's not the end of the world but it does consume more time.