I can’t speak to the ATC functionality, but they aren’t really working on Mach3 much, and the ESS plugin (I don’t know if you are using ESS or another controller) is only being improved in the Mach4 version.
I think AVID has left Mach3 completely behind as well.
One thing that is nice about Mach4 is it has a screen editor so you can modify your screens to fit your needs.
Here is a really nice custom screen example (for ATC support :-)) by Corbin. Custom Mach 4 ATC User Interface
Also, anyone interested in laser support will definitely want Mach4 since the ESS plugin for Mach4 has had a lot of improvements come in the last year or so.