A 6" vise will probably be overkill on your mill-drill - a 5" fits them pretty well.
Its probably a toss-up on "this-un bein better'n that-un"in the clones. It actually amazes me that they can sell that big a chunk of iron as cheaply as they do - and for that small amount the vises are really a pretty good buy. Yeah, they wont come up to a Kurt, but it'll get you by till later, -- maybe you can come up on a Kurt. (thats how I did it)
You may as well get the ENCO version and take advantage of the free frt, these things are heavy. The 5" is $92.95 and qualifies for the free frt.
If everything seems to be going well, you have obviously overlooked something........