In that case I would go for the zumo660, if you can afford it...
Easy to handle, can do most things and is also a good gps for the car.
Drawbacks compared to 276C are that small roads disappear when zooming out and some other minor things. In fact you can see 16 times more roadinfo on a 278 than on the zumo660!.
Almost all shortcomings for the zumo is related to the software, the hardware is superior to the 278 in all aspects. So new firmware updates could make the zumo660 a fantastic motorcycle (and car) gps.
There is an excellent comparision between the two gps'es but unfortunately only in swedish, maybe google translate can do something but it is a pdf. Here for your info anyway [Only registered and activated users can see links. ]