You've heard this before - it depends on what you like. (OK, disclaimer out of the way).
I would pick Carnival for the "up" atmosphere. While most all cruise lines are enjoyable the demeanor on the others is more reserved and casual. It's like comparing the general atmosphere of DisneyWorld to SeaWorld. Both are enjoyable, in their own way, but DW tends to have a little more "bounce in the step". The staff and the passengers tend to be in a more "up" attitude - it's the atmosphere Carnival promotes.
That said - this also reflects who I am. 51; but not ready to go on an Alaskan cruise until I "settle down" and want the more casual, enjoy the scenery cruise.
Here is something to consider:
The more the merrier - can friends go along? Granted, you will be on your honeymoon; you can spend time to yourselves. Good friends will give you your space. Sharing the cruise with others adds to the fun - dinners together, some evenings together and some by yourselves, etc. I have been on many cruises with some of the group on their honeymoon, 2nd honeymoon, etc. and we let them have their time and space.