resilience to sybil attack - public vs private it appears that although not totally water-proof to Sybil attacks, Bitcoin network seems quite resilient to these kind of attack Preventing Sybil attacks. Is it true to assert that permissioned or private blockchain are less exposed since there is a certain level control of the participant allowed to enter the network ?
Definate yes. 

The Sybil attack in computer security is an attack wherein a reputation system is subverted by forging identities in peer-to-peer networks.

If nodes are required to proove their identity before they can join the network, just like in permissioned or private blockchains, then they will not be able to forge the identities.