I am on my second Millrite and they are nice machines for hobby machinists. I know very little about the Rockwell, so I will leave it to others to compare them. The Millrite is definitely bigger and heavier. Millrites are well designed and built, just make sure it is not worn out and that everything works. I do not know of any weak points in the Millrite design, and it is a pretty conventional knee mill design. If you want specific information, feel free to PM me. You also might want to join the BurkeMills Yahoo group. Prices vary with location and condition. They are pretty popular, at least around here in California, so don't wait too long to decide or it may be gone. My first Millrite was pretty worn out, and I paid $1000 for it, and sold it for that later. My second Millrite came with some other stuff as a lot deal, so breaking out the mill price alone would be just guessing. I got an unused (new but dirty, no rust) 1967 Millrite with no extras, a like new Dumore 44-011 tool post grinder with the case and all the factory goodies plus more, an unused import angle drill press vise, and an unused set of 1/4-1" lathe arbors for $1600. That was from a friend, and I don't suspect you will be able to make that good of a deal!
Edit: There were lots of options on Millrites when new, motors, table sizes, knee size, power feeds, rapid or fine feed quill, nodding or just swiveling heads, and lots more. My new one has a 8x32 table, rapid quill, and everything else standard.