Crystal is good at everything, but the biggest difference in my experience is that their production shows are superior. I only have one Regent cruise for comparison (apples to apples), and what I remember was that my favorite Regent show was their crew talent show. The rest of the entertainment is generally very good on Crystal, but on a cruise just last summer, there were some misses for the first time.
The "at sea" days on Crystal are very full, and a lot of people pick Crystal because of that. (But while I enjoyed the at sea days, I would still rather be at a new port every day.)
As much as I've enjoyed my Crystal Cruises, I'm hoping to find the food and service to be as good on Oceania's Marina. If that's the case, then I think Crystal may lose a lot of business. They have no new ships coming online, whereas Oceania will have two. Crystal just refurbished Symphony, and while the changes are very nice, they can't do anything about the cabin size. So people like me that might book a regular size cabin on Crystal can get a Penthouse on Oceania for less.
But as, I said up front, Crystal is good at everything. You're going to have a great time!