It ends up as personal ( or team) choice. There's arguments for almost every gas mix. In my early days I'd have _only_ used 50 or 100% , now as long as it wasn't too deep I'd be more amenable to change.
For a lot of 'mid-range' deep dives ( I think ) a combination of 30% and 70% is actually fastest, whether or not it is best is debatable. In theory at least 100% gives the cleanest decompression - though it is not always available in sufficient pressure to be usable - hence the use of 80% in order to actually do a dive rather than pontificate over it.
I feel there is a lot to be said for using 2 stages of 50% for 60-70m stuff, the time increase over 50/100 is negligible, you are now fully redundant, and it doesn't matter which stage you actually use - ie it takes our the potential for muddling stages up. - one of my acquaintances managed to do a 70m dive and complete his deco ( solo) by switching to his 80% at 30m ( and stay for a few minutes to sort out something) and ending with his 40% at 9m - he did say something about 'bells'
The big problem with 50/100 as I see it, is that it then commits you to a true buddy pairing, as if the 50% fails for some reason, you are effectively screwed, and are dependant on a buddy to sort you out ( unless you really enjoy cold deco) In the real world, may of us choose to either 'same ocean' buddy dive or solo, which effectively rules this out as a combination.
Either way, there is a lot to be said for always using the same gas mixtures, and therefore always switching at the same depth ( I'm a simple soul, I like simplicity) preferably everyone in the team using the same mixes
