You should always try to use replication service unless you don't have enough time or if your quantity is small. In such case you can use duplication. The artwork for duplication service is usually printed on a paper label and then the label is stuck onto the disc surface. This is labor intensive task and that's why duplication service, despite the lower quality, is more expensive then replication service in terms of unit prices. I know a service provider called www.orbisdigital.co.uk, which provides both CD duplication or DVD duplication and CD replication or DVD replication.
Hope you will be satisfied by my suggestion.