A great advantage to Seabourn is that once you have sailed them you become a "club member" and qualify for 50% off selected sailings, of which there are quite a lot.
Club member value savings editing to add this comparison I made last summer after my first RSSC cruise:
Overall impressions of RSCC vs. Seabourn:
Ship/activities/entertainment – RSCC – bigger & more recently built means more venues, options, amenities etc.
Suites – RSCC - balconies, better closets & storage, separate tub & shower. Balconies are not a deal maker/breaker for me though.
Officer/senior staff accessibility & friendliness - RSCC
Overall service – Seabourn. Although the service is wonderful on the Voyager, it is almost unbelievable on Seabourn – they all know your name by the 2nd day, remember your every little preference.
Food – edge to Seabourn, but again it was excellent on RSCC.
Shore excursions – Seabourn – we did an exotic/adventuous itinerary (Indonesia) so maybe an unfair comparison, but shore excursions were in 8 seater vans, not buses.
Itineraries - Seabourn