We have mainly sailed with Azamara before which made comparisons easy since the ships are identical and came from the same original source. First of all, if you are planning a similar cruise to the South Pacific be careful not to go with preconceived ideas about how exotic it will be. I guess we all have notions about certain parts of the world from movies etc. And the places can't possibly live up to the hype. It was beautiful but so is the Caribbean. For me, only Rangiroa, Bora Bora and Tahuata met my expectations.
If you are traveling from Eastern USA or Canada then my recommendation would be to go early and spend a day or two relaxing in a hotel. It took us 3 days to recover from the long flights.
On the Regatta the food was mostly excellent - very similar to Azamara. The service was good although we did find a few of the crew to be rather arrogant. Of course you can find this on all cruise lines. Things like the dishes, cutlery, sheets and beds are superior to Azamara. First impressions are important! On Azamara they welcome you with a glass of champagne - on Oceania you get your photo done and see ya later. On Azamara a beach bag is on your bed no matter what level of cabin. On Oceania we were told "you are only in an inside cabin - we give bags to suite guests and above - of course you can buy one if you want" A $20 beach bag that provides free advertising for the cruise line.Give me a break! Needless to say that wasn't the start we had expected or hoped for.
Of course you don't go on smaller ships for the entertainment - it is usually mediocre. On the Regatta one nice touch was a string quartet that performed at afternoon tea and before dinner. They were super and we found ourselves going to tea just to hear them.
If you are into snorkeling or scuba diving then my suggestion would be to do some research and book your trips before you go - cheaper and according to the people we talked to better.