I would have thought there is no reason not to go with Mach3 as a new user.
The only reason I think that people stay with Mach2 is that they have been using it for
a long time and are comfortable with the look an feel of it and don't want to update.
Go with Mach3 and learn the current layout and features. Then it will be all the easier to keep up with developments as they occur.
Don't worry about Mach4 as that was to use with the Grex and now they have decided to do that with a plugin in Mach3 instead of having a separate version.
Hope this helps. Go to Mach's own support forum and have a read or ask questions.