I say both, natural speed and agility for my DE are both over 75. By Lvl 52 you should have both over natural 77.
http://goallineblitz.com/game/player.pl?player_id=844800Agility gets you by the OT, speed gets you the sack, you need both. Most of my DEs sacks this season are against guys his lvl or even higher, I think only 14 against guys greater than 4 Lvls lower than him. Forget SAs until you have 85 speed with EQ and 110 Agility. My team has DEs that were similar to mine I put SP into Speed and Agility, they went SAs, Speed and Agility worked.