i can say, i have two defensive ends, ohe with 16 speed, 11 strength and 5 agi and one with 11 speed, 15 strength and 6 agi. the one with speed gets almost every match at least one sack, the other one gets that rarely.
and also for safetys in my opinion speed matters, the faster they are the less yards wins a running back if he gets pass the first two lines.

maybe strength is much more important, but if anyone says to me, that speed doesnt matter, then i can only disagree.