I think Sanchez is an immensely talented individual that falls under the 'complete forward' banner. I'm convinced we could play him at CB and he would do a job.
Lacazette is by my understanding, less talented as a whole, but a specialist geared up for the striker role. certainly potential for him to be a better 'striker, no way is he a better player though.
The big question mark at this stage is if he handles the transition from league to league, if he does he could very quickly find himself as Frances #1 striker and playing with Ozil, perhaps in or around the league's top 2/3.
Certainly no Aguero or Suarez though. If you find @Cristo's post from the other day it frames the level of the top BPL strikers nicely, and i would take a stab in the dark and say he could be rubbing shoulders with Costa in terms of output.