If (at this moment in time) you just want the basic X - Y - Z milling then I would go for Mach4.
If you want the X - Y - Z and to incorporate all the air controlled functions (at this moment in time) then I would go for Mach3.
(Mach4 is coming along slowly but, in my opinion, it is not yet ready to fully take over from Mach3)
From the official documentation...
Software Requirements
Mach4 Recommended Requirements:
PREFERRED: 32 or 64-bit Laptop or Desktop – Using an External Motion Controller
• Windows XP, Windows Vista, Windows 7, and Windows 8
• An appropriate external motion controller
• Mach4 Plugin for the selected Motion Controller
• 2Ghz CPU
• 1GB RAM
• Video Card with 256MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
*Use of Parallel Port for machine control only with desktop style computers with 32-bit versions of windows 7 and below.
32-bit Desktop – Parallel Port Recommended Requirements
• 32-bit version of Windows XP, Windows Vista, or Windows 7
• Mach4 Parallel Port Legacy Plugin (cost applicable)
• 2Ghz CPU
• 1GB RAM
• Non-integrated Video Card with 256MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
Mach3 Minimum Recommended Requirements:
Requirements if using the parallel port for machine control:
• Desktop PC (Laptops are not supported) with at least one parallel port
• 32-bit version of Windows 2000, Windows XP, Windows Vista, or Windows 7 (64-bit versions will not work)
• 1Ghz CPU
• 512MB RAM
• Non-integrated Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
Requirements if using an external motion device for machine control:
• An appropriate external motion controller (options can be found on the Plugins page)
• Desktop or Laptop with Windows 2000, Windows XP, Windows Vista, Windows 7, or Windows 8
• 1Ghz CPU
• 512MB RAM
• Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
Tweakie.