My reason for not using the air II is simple I do not want to take my S600 out of my mouth and give it to a inst buddy who's rent -a -gear took a crap, I would much rather give him my octo. I prefer to keep my reg in my mouth at all times and if I need to help a buddy seems to me it would be easier not removing my air source.In the case of cave diving hose length may be an issue with the air II
I have donated gas in 3 real world OOA's over a 25 year period. One of them, the last one, was a more or less technically oriented diver during an ascent and I saw him coming so I had a deco bottle ready for him.
The other two were recreational divers I was not even diving with and both times I got mugged for my primary. Now this is not such a big deal as I believe in donating the primary anyway. But it is a bit of a shock when it happens and in the real world of semi panicked OOA recreational divers, the primary is going to get grabbed - but in most training agencies they still teach the unrealistic exchange signals and donate the octo approach to gas sharing.
The relevent points then are:
1. You can plan to keep your S600 primary, but your buddy is the one who will be using it if he goes out of air as he will go straight for the reg that is making bubbles as he can see it, he knows it works, and he wants to make bubbles of his own RIGHT NOW.
2. It's a good idea to ensure uour octo is at least as high quality as your primary as the odds are you will be using it in an OOA donation situation.
3. An Air II requires that you donate the primary and is oddly enough more in keeping with relality than the donate your octo approach taught by most agencies.
4. If you use an Air 2, a long hose primary makes things a bit easier as you can maintain a bit more distance and freedom of movement from the buddy. That makes up for some of the restricted head movement an Air 2 imposes.
5. A long hose is one of tose things that once you try you will never go back to a short hose - and it is consitent with the "donate the primary" reality of real world air shares.