I've been cruising a lot, since I was a kid, and fairly continually. Mostly ocean and with the mass market brands. Did one river cruise, and given the included excursions, didn't enjoy the "bus tour" feel when the ship stopped and the entire passenger compliment often boarded 50 passenger tour busses.
My 2¢:
We've never done the large ship "mass market brands" so it's a bit difficult for me to compare to them. But we have traveled many times on Viking and once on Seaborne.
The Seabourn ships are definitely smaller than the Viking ones, but to us they were surprisingly similar.
First the obvious differences - Viking holds 930 people vs Seabourn’s 450. But frankly, it we didn’t notice the size difference really. Viking certainly didn’t feel “bigger”.
The “main” rooms are different too, but again, more similar than different. Viking's Veranda staterooms are all 270 sq.ft. while Seabourn's Veranda Suite's range from 246 to 302 sq.ft. Seabourn also has a few 295 sq.ft. Ocean View Suites with no balcony (which we didn’t see).
We’ve never stayed in any of the “real” suites, but our last two cruise on Viking we bumped it up to their “Penthouse Veranda” which is 338 sq.ft. and provided just enough more room that we noticed the difference and felt it was worth the small increase in price to us. Our original motivation was on a one month cruise from LA to Polynesian and back (at the end of Covid) and we felt that for such a long cruise, the extra room would be welcome. Yes it was, so our cruise around South America last winter and next winter’s eastern med cruise was/will be Penthouse Verandas, so I guess we’re converts.
More differences..
There’s a small casino on Seabourn, not on Viking. This was a small plus for us on Viking, but the casino on Seabourn was unobtrusive. Obviously, your preference might differ.
Seabourn had a “formal night”. It was kinda fun, but frankly not our style. We don’t miss it at all on Viking. It means you need to pack something for it…
The style onboard is a little different. Viking is very much restrained Scandinavian, Seabourn a bit more formal, but not excessively.
Service was awesome on both. There’s no butler, but they do take very good care of you on both.
Food was excellent on both. I’d give a slight edge to Viking.
The excursions were both top notch. This varies a lot based on where you’re going of course. With both many excursions are a busload of folks (often the busses are only half full). We’ve done a number of excellent excursions here it was just maybe a dozen folks with us or less.
While the clientele on both lines was very similar, we noticed a few more people trying to impress others on Seabourn. Not to an extreme degree but it’s there.
We didn't use the Seabourn airport transfers, but Viking has always been top notch. They meet you at the airport and make the transfer as easy as possible. We've never had problem but one couple we meet had their luggage lost by the airline and Viking took care of straightening it all out for them. A bad experience was made "not so bad" and they still had a wonderful cruise.
Honestly, we like them both a lot. We almost booked a cruise for next winter on Seabourn - they offered an interesting Southeast Asia cruise, but the Airfare killed it for us. The business class airfare cost more than the cruise. So we’re going to the eastern med on Viking where the airfare is less of an issue.
Even though we haven’t done the large ships, I’ll offer what we like about the medium size ships ( < 1000 people - we haven’t done really small ships).
We mostly keep to ourselves, we’re just not interested in shows and lots’ of onboard activities which we understand is much of the allure on larger ships. We focus on the excellent onboard dining - mostly just us two, but sometimes dinning with other guests. We like the “lectures” and port talks which really bring a lot much improved understanding of the places the excursions are taking you.
Oh yes, there are no kids on these ships. It's definitely an older crowd. Our experience has been that all basically all the other guests are nice folks who are a pleasure to be around.
There's little jockeying for use of facilities. The pool areas are often not full. The "alternative" dining areas need to be reserved, but if you want to eat there more than your quota of reservations, we found it easy to go more often.
Not sure this will help you, but if you have any specific questions, ask away…