You should choose the duvet cover with the hidden zipper placed at the edge/back of the cover.  There are 3 advantages to using hidden zippers on duvet covers:
1. A regular zipper（not hidden）placed on the surface/edge of the duvet cover can break or weaken the sense of luxury of the cover.
2. Hidden zippers are covered with a layer of silk which makes it less likely to cause abrasions on your skin.
3. The cover that protects your skin also protects the zipper from breaking, granting longer-lifespan to the zipper and less trouble for you.