Doesnt make a big different actually. Many datacentres make a big fuzz about offering 100 Mbps uplink over 10 Mbps for an additional fee. But all servers on a datacenter are on shared internet lines, and unless you are doing some heavy ftp or file transfers during server migration, from server to server on the same data center, it will not make much difference at all.

Any regular user who wishes to view a website on your server will not take more than 1 KB of traffic at any given time. So even if there are 5000 visitors on your server viewing all the sites, you need a bandwidth of only 5 MB. And that kind of traffic a single server cannot handle, as you will hear the hard disk exploding if 5000 requests are given at a time to the hard disk.

It is not really necessary that you go for the 100 Mbps connection. If you think it is required for a migration or something, you could get it temporarily for a month or so, and roll back to 10 Mbps while your servers are in regular operations.