Mill-drill has a larger work envelope, but the round column is a bit of a negative.
X2 is barely big enough to almost complete an AR lower 80%
The mill-drill is big enough, but you will probably have some setup issues due to the round column. Nothing you can't work around, and infinitely better than a drill press.
Ideally, you will step up to a bigger mill such as the X3.
Used, look for a Rockwell or Millrite. These are small knee mills, can be found for about $1000