Orange peel, whether "sweet" or not will naturally have some bitter characteristics to it because of the natural rind of an orange on the backside of the skin. But the sweet stuff shouldnt be nearly as bitter as the "bitter" peel, obviously. I just kegged a belgian wit with sweet peel and grains of paradise without any bitterness attributed to the peel. 15 minutes is a long time, try 7 to 10 just to extract the flavor. Boiling anything too long extracts lots of bitter flavors. Think of tea, if you steep it for 10 minutes it will be really bitter, but 4 minutes makes a nice balanced cup, the same rule applies with beer additives.