This is a toughie question for me, but, interesting. I have sailed on HAL trans-Atlantics and enjoyed them. Not the party-hardy group of guests, but, plenty of activities during the day and evening that I never became bored.
The inaugural sailing of Carnival Jubilee would very much interest me. Sailing on such a voyage has long been on my bucket list. I would expect that there will be many Carnival-cruise type activities offered, but, there is likely to be some special events because it is an inaugural sailing.
Surely, there will be a difference in the number of guests on both ships, but, long voyages like trans-Atlantics often don't sail close to capacity. 5400 guests on Jubilee? I'd be surprised there will be close to that many.
Another thing to consider is that this crossing is Jubilee's first revenue cruise. If so, new ships sometimes have initial start-up issues that can be bothersome to some guests. However, since Jubilee will be the third ship of that Class, Carnival will have learned from Mardi Gras and Celebration's experiences when they entered service. That should make for a "smoother" entry into service for Jubilee.
I have sailed on Carnival Freedom, Liberty, Pride, and Miracle. None of these are as large as the Mardi Grass Class is. "Will we get the feel"? As far as the onboard atmosphere, yes, I would expect so. But, the size difference? No.
What a delightful decision you have to make!