This is like comparing peanut butter and jelly (pun intended XD). There really isn't a best answer unless we know the other Pokemon. Excadrill is a very good Sandstorm Sweeper, able to rip through many things. On the other hand, Landorus can Sweep many things regardless of whether (again, pun intended) there is Sand up or not with it's large Attack stat and a Choice Scarf. It's power is also boosted with Sand Force.
Overall, I would use Excadrill if you aren't using Smogon tiers and you want a more defensive Sweeper. I would use Landorus if you want a powerful Revenge Killer and a straight up Sweeper.
