Whats best on a sat nav?

fastest will use major highways and can add to journey mileages. based on high speed roads use. shortest is based on reducing mileage between two point so will often involve convoluted route. for novice users ...fastest is best.

Fastest, unless you want to end up in a farmyard or someone's back garden. The shortest route isn't always the quickest

I would suggest looking at a paper map or on Google Maps and coming up with your own route. The fastest route assumes empty roads, and might be longer distance-wise than the shortest route but on faster roads. The shortest route is the shortest distance-wise, but being on slower roads might take more time.