We have always enjoyed Carnival but are getting very tired of the "same-old, same-old" routines and shows.
That's actually probably the very best reason yet that I've heard of to change.
The other differences between the lines may seem minor compared to that.
Only problem is you'll have to open the wallet a bit wider to book RCI.
You may want to take a boo at NCL as an alternative too.... any of their newer ships... which I think applies to RCI too, because the older RCI ships have pretty small cabins.