There are two basic reasons to select one neck over the other.  One is to find the right notes or chords, the other is to find the right sound in terms of pitch.  As I develop more knowledge of the tunings I use (which are very similar to yours) I find that I'm able to play the same notes in all tunings, so the choice for me mostly comes down to pitch.

Selecting for Pitch

The thickness of the strings in the different tunings, lends itself to a style you are wanting to produce. I think of my higher pitch tunings to be more of the pretty ballad and fast tempo type songs and the lower tunings to be more for the swinging/bluesy/melodic type songs. Now that's just the way I hear it and like to separate the way I interpret and want to feel the song played.

Selecting for Notes/Chords

However, I do break my rule quite a bit because I like to hear certain chord structures being played and I know where they are on one tuning and I might have a harder time finding those intervals and the way they line up, for another tuning and that's the luxury of having different tunings on a multiple neck steel. So all and all, you can produce the music you want out of all the different tunings and there really is not a rule for which tuning fits what style song.

But I do like to try out what I'm working on one neck and then play it on another neck and hear how it lays out and sounds and that's what makes me decide what tuning I would use for all the styles. I believe if you can play something on one tuning, you should try and play the same thing on another tuning and with that you will find what sounds best to your ears. If it sounds good to you, you will more likely play it with conviction and more confidence simply because you like what you hear.