I got the double so I could do a modified mitchell system. I've ended up using Gerry B's system so have only used one of the rollers at a time. Nice feature of the double is that depending on how my climbing hitch rides I can use either the right or the left roller to allow better feed.
Someone mentioned earlier that the offset induces additioanl friction. While that is theoretically true and the offset visibly noticeable, I've not noticed any difference or difficulty from it.
However, now that I've used the double I wouldn't hesitate to save $$$ by buying the single.