In Ck2, as a catholic, you play a noble. Noble are supposed to hold only castle.
If you hold a city or a bishopric, you'll get a "wrong holding penalty" lowering taxes by 75% and maybe some other stuff.
For the pros and cons of getting baronies you have :
pros :
- if your territory is little, grant yourself more direct tax and full levy
- If you put a chancellor and have multiple holdings in the county, they all get boosted (very very efficient if you hold a 6 barony county.)
cons :
- If your territory is big enough, it grant less than having another county (less direct vassal, less technological spreed inside your demesne...)
My opinion is that you must always think about the difference with taking a county if available.
If there is no county that you can take, there is a very high chance that getting the barony is a good thing for you (if the demesne size allows it).
If you can take a county, there is a lot to think about : technology, any vassal duke having de jure right on the county, what is already built, your current relation with each vassal...