EZ,
I think it depends on what you are going to be using it for. Although this is the "Tech" section of the board, I'll comment on my experience with single tanks just to give you another opinion.
I switched to a long hose about a year ago. I started out with a 7 footer. I was still using a jacket BC at the time. Hose routing from the first stage was down the right side of my body, hooked under the right front of the BC, across my chest and behind my neck. This seemed to work well for a while...but, without something more stable to hook it on, it had the tendency to slip out on occasion. Lots of folks recommended slipping the extra into your belt....tried it, didn't like it. Others I've talked to have used a pocket on their belt or even mounted a piece of PVC pipe to act as a pseudo canister light (and storage container). I felt getting a different size hose made more sense....especially since I am diving only single tanks.
I ordered 5 and 6 footers just to try them out. 5 turned out to be the best option for ME....routed directly under my arm from the first stage, across my chest and behind my neck. The hose does not inhibit head movement at all (I am not a small person). 6' was too short for the 7' routing and too long for the 5' routing.
One thing to consider.....with this 5' routing, your hose is now potentially running through the same area as one of your back-up lights if you are diving a DIR configuration. If you take out this light during the dive, you have to be somewhat careful that you dont't clip the light over the hose when you restow it. Since this routing is generally for rec diving, one solution is to only wear a back-up light on your left.
Hope this is helpful.
-LD