As a developer - testing for WinRT?

So, I am currently deciding on purchasing the WinRT version or the Pro version. I am outsourcing various development and I wonder if it makes a difference for me as a tester which version I am buying.

One example is that I am planning building a WinRT only app - will that app automatically work on Windows 8 Pro (in "Metro") or will it only work in Metro for WinRT version? If so I only need to buy pro version - otherwise I need to buy both. Please advice