I've not been on Regent, but I have sailed on Oceania two larger ships Marina and Riviera. I've also sailed on the SIZE of the smaller O ships, but on Azamara.

I sailed Seabourn ( down the coast of Chile and Peru in 2019

Sailed 16 nights to Antarctica November 2021 on Silversea.

Just did Silversea Galapagos in August.

My 2 cents. I enjoyed Oceania food ( specialty restaurants, no charge) the most of all the lines. The pax # a little bigger than we like on O, but it was fine. O also has a little thing to most people, but to us a big deal. Self serve coffee machines. Also, we don't drink, so not having alcohol included was nice that we weren't paying for something we didn't use. SS and Seabourn were expeditions, so those excursions being included was a big deal. Suite size ( regular balcony stateroom ) pretty much a tie. All are fine.

Ask me any questions.