All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
These two Islamic terms cannot be compared because there is a massive difference between them. A person who is weak in faith remains within the fold of Islam because he still possesses the principle faith, but his faith is not perfect. This is because faith fluctuates; it increases by performing acts of obedience and decreases by committing sins. However, Kufr (disbelief) is when one is totally out the fold of Islam.
A person with weak faith still has hope to be forgiven and is still entitled for intercession if he dies upon this state. Even if he is punished for his sins in fire, yet he will eventually be admitted into Paradise.
This contrary to a disbeliever who has no chance for forgiveness or intercession if dies upon disbelief, and will dwell eternally in the Fire of Hell; Allah Says (what means):
• {So there will not benefit them the intercession of [any] intercessors.} [Quran 74:48]
• {They will wish to get out of the Fire, but never are they to emerge there from, and for them is an enduring punishment.} [Quran 5:37]
Allah knows best.