When do you want to have your uber power and who are you against are better questions.
#Explination:
Crit is a later power spike but is far superior to lethality in almost every way, especially since it is effective against almost if not all opponents. The only trade-off being a delayed power spike.
Lethality is best when against squishier targets and has an early power spike while the trade-offs being not having the same insane damage as a full crit build, no CDR, and you have to have boots which is 1 less damage item.
I would always recommend the full crit build if you ever are unsure as to which build would be better since it is always better in the end simply because it scales insanely well. But if you know your enemy is gonna be squishy, then lethality is better because of the low armor your targets will have. Lethality offers more resistance too, but usually you can work through the fight without it if you are careful/familiar enough with him and the enemies' champions.
Both builds are fine in most situations, but the more tankiness/armor the enemy is/has, the better crit is over lethality.
#Builds:
Crit:
/
(pick one, either works depending on situation)
(replace with
once you have the 3 crit items/70% crit)
/
(I actually go shiv since it adds some magic damage wave clear to your burst and can also crit, but the standard is RFC)
/
(the one you didn't take first)
/
(pick one of these)
- Honestly your choice here for a 6th item but I would recommend
or 
Lethality:




/
(pick one of these)
or 
#Side note:
I would not build
since only basic attacks over heal you and you have 4 shots at most at any given time. Plus with the buff to BORK it is imo a better option for lifesteal, or just take the usual Death's Dance.