I am looking at those cruises too. Having just been on Arvia, she is a fab ship with plenty of things to do, places to eat... and, importantly to me, 16 more whirlpools than Britannia! However, the latter does have the more appealing itinerary.
I have a dilemma though. I am due on a short cruise at the beginning of June, so still within the 'introductory offer' period. All being well, I could book on board with lower deposit and higher inboard spend. But the risk being that lower grades sell out, or they simply put the prices up beforehand. Decisions...