Well, it depends only in the price.
I always prefer machining centers than "converted" milling machines.
I got my machining center for $3000 .
If you are learning, I would recommend you to buy a machine with newer controller, with screen and text menus, so you can see all the information the machine is working with.
Something that bothers me, when using my machines, is not having a "go to" reference, so I can know at every step of my program where the machine is, and where it is moving to. Without that reference, I have to be sure that my program is ok before I run it.
Also, it is most useful to have a complete Z axis automatic movement. Once I put all the tools in the machining center and put their Z reference, I forget about them and focus on the program, because I know that all the tools can reach the piece of a given setup, the table will always be at the same distance, because all the head moves with the tool.
But for a knee mill, your automatic "Z" is smaller, and sometimes when you must to raise or lower the table in order to use a short tool and a longer one, in one program.
So, if its really cheap, go ahead and buy some fun.
But if you can find a machining center for the same price, better get the machining center, or buy/make an inexpensive cnc router, less rigid, but with the same characteristics of any CNC.
By the end of the day, the only thing you need is to learn in G code: How to make a line, how to make a circle or an arc, how to compesate for tool size, how to use feed and speed. Then to learn in how to make complex surfaces in a 3D CAD/CAM software and make it generate de G code.