MSC appears to offer a generally higher end cruise than Costa does. At least the reviews appear to indicate this and Berlitz rates MSC number three, (Below Cunard #1 and Celebrity #2) in food, drink and service out of the eleven largest majors.
You might be ahead if you are multilingual as a guest on either one of these lines. MSC makes announcements in five languages, English, French, German Italian and Spanish, the announcements however are few and far between after the cruise gets underway. MSC provides no lecturers because of this and production shows are mostly visual not verbal. On Costa's Mediterranean and European cruises English is the least spoken as most passengers are Italian, Spanish, French and German. On the Caribbean a large percentage of guests speak Spanish since Costa really caters to Latin Americans. Lots of children on Costa and lots of partying. They say if you want quiet on Costa, bring earplugs, good ones.
It is my understanding that MSC is building three vessels for North America. Sounds to me like those cruises will be adjusted to reflect North American tastes more. Costa has to be the most smoker friendly line of all the majors today, as I mentioned, they are the only one who allow balcony smoking today. MSC prohibits balcony smoking but does cater to cigar smokers and has exclusive cigar lounges.