I'd get the biggest vise the machine could handle. I have a couple of cheap 4" vises that I sometimes mount with one on either end of the table, used to hold long pieces. Their very handy for that, but not large enough to do most general purpose work. I would consider a 5" as minimum for most work I do, but that's me and my situation. Most good mill vises let you mount the jaw plates on the inside of the jaws and also the outside of the jaws for larger pieces, might want to see if the brand you mention offers that feature.
Realize that most common vise accessories - parallel sets, spare jaws, etc., are designed for 6" vises that meet KURT standards. Not a big deal to have a set of parallels stick out from one or both ends of your vise, just want to point out what's available and cheap.
I like vises that don't have protruding mounting feet. Handy for clamping down the vise in its conventional position but awkward if you ever decide to mount the vise on its side. I like flat sided vises that I can clamp down on either side as well as the bottom.
Don't let making a perfect decision stand in your way of making a good decision. No matter what you get you'll figure out how to make it work for you. Like trying to buy the perfect rifle, there is no such thing.
