I have the 660 & use it with a Nokia N95 mobile 'phone.
I use an interphone F4 - connects via bluetooth to both the SatNav & through it to the 'phone.
I think the Scala is very similar.
For me the choice between these two would be dictated by my choice of helmet - the Interphone speakers are thicker.
I use either an AGV S4 (2008 version) with the full-face kit, and the speakers are a bit of a squeeze (I think the cheekpad style has changed in the 2010 S4 version, and is probably a bit more accomodating) & a Roof Boxer with the open face/flip-up "universal kit" which works OK when the front is up & over, but there's not really enough room for the mike behind the front plate when its locked down.
Tomorrow is my **th birthday & hence I will be fitting the universal kit to a new Shark Evoline to replace my tired Boxer.
As far as I can tell both the Scala and Interphone are pretty similar - I bought the Interphone as most reports seemed to suggest that it had better sound performance - probably as a result of its thicker form factor. In time I plan to get a set of Ultimate Ear monitors (or some such).
Good luck