Six and two threes.
Timewise there isn’t a great deal of difference, if you go via the A303 then start off by heading up the M5 and then going down the A358 rather than going up the A30 from Exeter which is very steeply graded.
The A303 obviously requires a lot more input but it is a good driver’s road with far more places to stop, more facilities etc.
I’ve done this run hundreds of times and tended to prefer the A303 unless I was feeling particularly lazy.