Where exactly are you going OP ? And from where ?
I live in Surrey, on M25. My best frind lives in Torquay, Devon. And I originally come from Devon, and my mum lives near Oxford. My brother lives near Bristol.
So , apart from giving you my life history ( !!) I was trying to explain that I travel alot, all over the place, on the M25, M4, M5, A303 for years.
I agree with the other posters. Whilst the A303 is prettier, and stonehenge is good, it is a truely awful road. And i nearly always get stuck behind some tractor or something, with just nowhere to pass.
The M4, M5 always 'seems' twice as quick. although its not actually. But then i actually like motorway driving. I drive reasonably fast, normally in the outside lane.
what kind of driving do you like ? if you quite enjoy motorway driving, as opposed to scenic, I would STRONGLY recommend the M4 M5.