ok, living in penzance and coming from hayling island i have driven up and down a million times.
both routes can be a nightmare at peak times so that is a given, however my longest journeys have always been the a303 route.
beyond that it really depends on how you like to drive. fast then i would say the a303, there is a trick through Salisbury though and basically take the a360 up to the a303 not the signed a36. its a faster better road with less traffic.
after that follow the 303 to the big roundabout at podimore and then take the a372/a358 which gets you onto the m5 faster. if you follow the 303 its more of a drag.
stop at exeter services for the m&s, cheap but good sandwiches, and you are pretty much there.
however, my preferred drive, actually shorter and can be a total pain in peak traffic as overtaking is limited, is the south coast route a35/a30 via bournemouth, dorchester, bridport, honiton etc. onto the m5 at exeter airport and so on. just a nicer mellower drive i think.