The 5" one at just over a third of the price will be more sturdy than your mill. I have the 6" version of that vise. My 5" vise is essentially the one that I linked you to, except Taiwanese brand name. and you don't have to use the rotating collar, which is then essentially the same as the 4" but bigger.
The lock down ones are nice to have. At 300 bucks there are other options, however.