Hi
Peer to peer gives you a reliability advantage, if someone lags out, you can just drop them from the game if you wish, with one person being the server, if they lag out, everyone gets disconnected. The down side is security, with peer to peer, everyone knows everyones network address, which means a players can do nasty things to one another, unless thats part of the game play?

peer to peer also introduces firewall issues to a large number of people. UPnP *might* help you on this issue, but lots of people disable it on devices that support it.
HTH
Endolf