We have sailed on both and it's a close call. They both offer a very special experience, however, our first choice is Oceania. My husband has celiac disease (an allergy to gluten) and Oceania is extremely accommodating with his dietary needs.
We decided, at one point, to try Regent. It wasn't a good idea. It was as though no one on staff (including the chefs) had ever heard of a gluten-free diet. When you would ask if a particular dish might have gluten in it, they never--not once--could answer the question and often even seemed annoyed to be asked. He ended up eating a lot of steamed veggies and grilled meat.
To be fair, that was a few years ago and I would hope the situation is better nowadays but, even prior to that, Oceania was handling special diets seamlessly.
Needless to say, we have stayed with Oceania ever since