Thak you for your input. Entertainment is not a major factor on any Oceania ship - never has been. The theater is far nicer than any ship they ever had before, but the ship still does port-a-day itinreraries like Oceania's other ships - and most people will probably go to bed.
I do not recall the short spaces on Marina - in fact I personally recall the opposite - the use of space for elegance; wide hallways and taller ceilings in public areas. But I am sure you recollect correctly, if you could be more specific.
The staterooms are smaller - but I personally wonder why that is a big issue. I mean I prefer a larger stateroom, but when you think about the fact the stateroom size is the only thing that separates the $2999 passenger from the $7999 passengers ($6000 vs $16,000 couple).
Realize that everything else on the ship is the same for everyone; food, entertainment, shopping, casino, ports of call, etc - and suddenly spending twice as much just for a little more legroom seems a bit misdirected. I realize this is something of a unique theory, but it is mine and it makes sense to me.
I feel Regent may attract for different reasons - more inclusiveness being the main one. But Marina is a surprisingly beautiful ship, and I think value becomes real when looks, service and cuisine are as good as can be. You may still get more on Regent or other luxury cruise lines, but is it really worth the difference in what you pay? I think you have to try it to see.