I have talked quite a bit in terms of the different VHDs there are, let's now focus on the different controller types that are available.
When you create a VHD, you need to attach it to an emulated controller in order for the VM to use it. You have two options: SCSI or IDE, but which one should you choose and why?
The rule of thumb seems to be: if you can use SCSI, use SCSI, but why? SCSI emulated controllers allow concurrent connections on the BUS, so this will make operations with multiple VHDs faster. Furthermore, your VHDs can be bigger with SCSI controllers (2 Terabytes SCSI vs. 127 GB IDE!). If that was not enough, SCSI controllers have more devices that can be attached to them. IDE supports only 4 connections, SCSI supports up to 28!
Furthermore, emulated devices such as DVD drives can only be attached to IDE buses, so you better save those buses for these types of connections.
So when should you use IDE controllers? Use them when working with removable media or when you are dealing with a VHD that has an OS that will not allow itself to install itself on a SCSI connection.