I had read up on this and decided I didn't want the injection and wanted the placenta to be delivered naturally - but when it came down to it, I couldn't care less. I had my baby in my arms and I just wanted to get the 3rd stage over with so that I could get up on the bed and get more comfy and snuggle my baby. So I agreed to the injection.
There are advantages and disadvantages to both options. You can google some great articles that will help you make your decision.