How does retiring EUAs differ from retiring CERs?

There are two key differences.

1. Retiring EUAs reduces emissions in Europe, while retiring CERs reduces emissions in developing countries. See What's the difference between a Certified Emission Reduction (CER) and an EU Emission Allowance (EUA)?
2. Retiring CERs raises questions about whether the funded projects would have happened anyway. This lack of certainty about the 'additionality' of the projects has been acknowledged by the Clean Development Mechanism Executive Board, the committee which oversees the creation of this sort of carbon credit. EUAs are not generated by projects and retiring them is not open to the same challenge.