What are the benefits of choosing a higher difficulty in Resident Evil 6?

I started playing Leon's campaign in Professional as I always choose the difficulty before last...I would like for these questions to be answered please.

• What advantages do harder difficulties provide ??
  • More skill points ?
  • Achievements ?
  • Unlockables ?
• How much harder will it be to kill NPCs ?

Solution 1:

There are achievements for completing the game in every mode except for No Hope. So completing game on Professional will unlock all of these achievements at once.

Number of received skill points is doubled for No Hope difficulty (source).

All other unlockables are not related to difficulty setting.