Ursa in a nutshell
Ursa hits like a train full of bricks and can do ursa as soon as he has vlads. His weakness is his ability to be locked down. Blink dagger cures him of his initiation woes. Shadow blade can also serve this role and give some damage, but takes longer to get and isn't as effective against smart opponents.
Sheepstick late game:
Cures ursa's mana problems and gives him a monster disable that almost guarantees a kill every time it is used.
Strength late game:
Lacks a the disable of sheepstick but provides more raw damage while also boosting ursa's health which gives him great damage during his ult. Mana can be a problem if you're spamming overpower to farm faster.
TIP: If you're having trouble last hitting in lane, hit them while they're relatively high on health once so that you get the damage bonus from frenzy to last hit with. Makes it very hard to deny creep from ursa if he does this but also pushes the lane faster.