I bought the SPIII over the Navigator coz I got a great deal with Staples, but having said that, the additional bits you get (bean bag etc. are great.
The power lead can be bought from BMW (very easy to install) cost about £35 (pricey I know, but worth it for the lack of hassle). The bracket costs about £40.
I landed up buying a BMW bracket off a friend who wanted to upgrade to the Touratech model, so now I have a complete SPIII which can be used in either the car or either of teh two cars (wife or mine).
If you buy the Navigator, you'll land up buying the cigerette lighter cable/speaker and bean bag mount for the car anyway (probably!!)
On balance, the Navigator has to be a silly option?!