It all depends on alot of different things. What type of work are you going to be doing with it ( steel, plastics, aluminum, etc.). High precision work or just milling in general. If you are going to be doing heavy milling or large parts without having to relocate then a RF or even X3 may would be a good choice. With just learning how to do alot of the little things that goes into machining the round column mills can pose more setup at times. With the round column mills you mainly use the quill for Z-axis or Depth of cut, or you lower the head. The general rule of thumb is the less of anything extended below the mill head the more solid, less likely to chatter (vibrate) and inturn lead to higher accuracy work. Everytime you move the head on the round column mills you will need to reindicate the head to the work whether it be a vise or the actual material in setup. With a square column mill like the X2 you lower the whole head for depth control and it is mounted in dovetail ways so it stays set for the most part.
Either choice would be fine I looked at both before I purchased my X2, which was at a really good deal. The bigger mill will obviously require more space, power, and a hefty table to bolt it down to. And if you want to change its location for whatever reason you better call some help over.
Another option you could do is to buy a X2 or X3 mill, get some training videos from Little Machine Shop, read everything you can on here and on other places like mini-lathe.com (they have mill info also). get some aluminum stock and start machining some practice pieces up. Then you can buy a CNC setup and make all the brackets and other odds and ends yourself. That would save some money on the CNC costs, give you some experience, and if you are trying to troubleshoot something down the road you will know "hey there was a spacer in this part and now its missing thats where the backlash came from." Just my 2 pennys though, you should get alot of opinions on various ways to go about. Welcome to the addiction.