Purely personal view (no axe to grind over the whole Windows/Linux shouting match) - for the kind of work I do at present, not much difference at all. I admit to only playing with a demo copy of Mach3 so far, but I shall be migrating to that as my new machine will use a motion controller that doesn't work with LinuxCNC. I helped someone get Mach3 up and running on a new machine once, and even though I had to do a bit of web browsing to find out where various bits and pieces and parameter settings were in the interface, the underlying functions were the same as LinuxCNC. For vanilla out-of-the-box operation, they both do all the basics. I am only moving to Mach3 because of the motion controller support, although the availability of various add-on options (different screen layouts, additional interfaces for controlling probing, things like that) seems to be much better on the proprietary Mach3 system than the open-source LinuxCNC. The garage PC only runs the router, so apart from LinuxCNC/Mach3, all I do is copy files over the network from my main server system, or occasionally use a web browser or play radio recordings (but not while cutting). Not a lot to choose between the systems for that kind of thing, once you have them set up.

The differences, where they exist, might be a bit subtle for the new user looking to choose. For example, one relevant difference for me is that Mach3 supports proper dual-motor homing on a single axis, where LinuxCNC (surprisingly, considering how long people have been talking about it) does not. But even that functionality varies once you add in an external motion controller. People can and do argue for hours about which is the "nicest" user interface but frankly, you can get to grips with the basics with either, or look for add-ons (or even write your own) in either system if you are that way inclined.