What are strings named after modes? - how do I know which to use?

Hi,

I have 2 sets of McSpadden strings, but I'm confused as to which ones to use - one set is called Mixylodian, the other is called Ionian. The differenceis something to do with the gauges, right? Anyway, I usually play in the standard DAD, but will also retune to other combinations. What packet of strings should I use?

Thank you, Ruth