If you attack from the 3-9 line you don't need as much speed/altitude.
If you are in a cannon armed plane, the 3-9 attack is probably preferred. In a U.S. fighter armed only with Mother Deuce, I prefer the high-six attack as Kazan so wonderfully described, so you can target the wing root/fuel tanks. In Spits, 190s, etc. I have more success with 3-9. With the Jug, Hog, Pony, I always use a high-six. My aim point in the dive is the nose of the lead aircraft until I open fire, as this drops me in right behind the tail of the formation, giving the tail gun position a very fast tracking profile. Likewise for the 3-9 attack, track the nose of the lead aircraft, and once you open fire, don't pull any more lead, so you can slip behind them quickly without risking a collision.
Perhaps Snailman will post his preference, as I know he's a bomber-killer of fame...