We have been on most major cruise lines, ended up loving Princess the most and have been somewhat loyal to the brand lately. We do venture to other lines (did NCL and Carnival last year, trying MSC next year for the first time), but nothing beats the feeling of stepping on Royal Class ship (our favorite class) for us.
Just an FYI, MSC matches status (one of the reasons we decided to try them), so Elite on Princess will be matched to the highest level on MSC.