netTcpBinding or wsHttpBinding

You can expose your service over more than one binding if you wish, so you could actually use both.

However, if you control both client and service and they both use WCF, netTcpBinding is much faster. Unless you have a firewall between those two, I would choose that.

Check this out for a comparison of all the different built in bindings:

Configuring System-Provided Bindings - MSDN

As for your case, as long as it's the web server contacting the WCF service and you don't need to provide an endpoint for any external consumers of the service...netTcpBinding should be up to the job.