Go with what works without problems or special considerations... people can weigh in if they have information to the contrary
I have both versions installed for: (associated modules in brackets)
- Python 2.7.x ... aka ArcMap up to 10.3.1 (Numpy, MatPlotLib, appropriate python versioned)
- Python 3.4.x ... aka ArcGIS Pro to version 1.1.1 (Numpy,SciPy, SymPy, MatPlotLib,Pandas...etc blah blah)
Amazingly, if you let Arcmap and ArcGIS Pro install the version of python etc FIRST, PythonWin and PyScripter find them and their appropriate version without any floor dancing about paths etc.
So unless you have a mission critical reason to use something else, you might use the current known-to-work IDE's and wait until IronPython comes along (or so is rumoured)
Back to you Curtis for counter-point