I would go with the reasonably inexpensive, but very reliable Oceanic Omega II. If money is not an issue, then the other more expensive side breathers are great, but an octo is going along for the ride most of the time, not being used as a primary.
I have used the Omega II for about 10 years for diving, and also use it when I teach courses because it's very easy to grab and not worry about which side is up, especially when my eyes are on the student who is doing regulator recovery with no reg in their mouth. I simply grab it and pop it into the student's mouth if they can't recover their reg in time. I have done this many times without fail or hesitation. The look in their eyes when they take that first breath is always a pleasure to see because the anxiety for them starts to subside immediately. A student will take the reg from you and put it in their mouth very quickly when they need air fast, sometimes without checking to see if it's right side up or not. After seeing this happen a couple of times and watching them try to breathe a wet reg upside down, I started using the Omega II. You never know what you or someone else might do in a situation where the octo is really needed. Students still have to learn how to use their own primary reg and octo setup though. The Omega II is a nice, safe convenience for me when I'm responsible for them.
Sometimes I will switch from my primary to my Omega II when taking pictures, just to get the bubbles out of my view for a while. You'd also be surprised at how quiet it gets underwater when you aren't listening to your own bubbles blowing by your ears! The two Omega IIs I have seem to be bullet-proof. They breathe a little wet for a primary, but are almost perfect for an octo. They will also fit in a bcd pocket if you can't temporarily re-attach them to your octo holder while swimming.
Just my 2 cents worth. Hope this helps.
