In the end it's really up to you ....................if you just want some extra go, fairly simple installation, AND supercharger whine ...........go with the single, it'll flow plenty for light mods .........BUT if you really hate it's looks so much (since the hood is mostly closed) then spring for the double, BUT it's twice, if not more, work to fit, twice as expensive, and unless you're planning manifold mods later, may not give you twice the bang for the buck .................it obviously flows better, but without further mods will you get the best out of it ?
At one time Rob used to let you upgrade later if you buy the single, but I don't know if that's still true.
I've had the single for two years now, and am totally happy, and it only takes an hour or so to fit, hardest part is getting the plastic piece out from beside the radiator ........... :O)