The bearings are important (and SON will tell you what they use while SP will not), but there's another reason entirely as to why SON hubs last longer.

As the dynamo draws power it generates heat which causes pressure to build inside the hub. If the hub doesn't have some mechanism to pressure equalise then it'll try to pull/push through the bearings, which depending on what's around could draw in stuff that doesn't work well in bearings. SON hubs have a balloon mechanism to keep the pressure the same and prevent this from happening.