It is a general question so let's answer equally generally:
A P2P network structure once established will always have the potential to consume less bandwidth than a client-server approach. To see this just think that the client-server is one given structure (clients all send their data to the server and the server responds back). P2P can generally form any structure including the above if it thinks it is good for what it wants to accomplish. So it could always be at least as good as C/S or better.
On the other hand a P2P network will have some overhead to form and maintain its structure. So if its life is long enough (e.g. a file download is large enough) only then is the gain worth it.
Regarding bandwidth let's also think that consuming bandwidth is not a problem if enough bandwidth is available (the cables do not mind if data is passing through them or not). Consuming bandwidth is only a problem when there is not enough bandwidth for everyone. This brings us to a more important benefit of P2P. Client-server connections create bottlenecks as more traffic is concentrated the closer to the server we get. A P2P structure typically aims to avoid traffic concentration ponts, not to mention single point such as a server.
There are also more considerations than bandwidth you should take into account when selecting a P2P or C/S model.