Neither. Armor pen reds, movespeed quints.
Quints are pretty preference-based though. I like movespeed for the added chase and harass potential in lane and for escapes - mobility in general is really strong throughout the game and there's typically only so much of it you can get in your build, particularly as an AD carry or assassin. Assassin especially needs to be able to close the gap and get back out fast a lot of the time.
Lifesteal quints, eh...they're good for sustain in lane, but later on that extra 6% (assuming you use all 3 quints for it) added to your bloodthirster/hydra probably won't be as impactful as being faster than whoever you're chasing/running from.
Could also go with flat HP quints to tankier early laning but, again, not so important late game, where speed is great from start to finish.