For the co-location you can build your own server and send server to datacenter. They will connect it in networks as you required. For the dedicated server, severs are already build and it is in network with other server. When you place the order they will send you the userid & password.
For the Colocation you will have more control over everything such as networking hardware and server hardware. If you are going to host around 15 -20 server then colocation might be a good choice. If there is any failure on your hardware then datacenter staffs won't help you much to fix.
Dedicated server is best solution for low amount of servers and datacenter staffs will assit you on hardware problem as well.