The 660, like the 550 before it, is a great bit of kit.
If you have no use for the iWheel and / or the data about your awesome that the BMW device can display, stick with the 660.
Interesting (or not) when the 660 was launched it had a number of teething problems; all since cured. The problems were though sufficient for end of production line stocks of the 550 to sell at a price higher than the 660 for quite a while.