Pros: It would look cool and show off that you have money (bragging rights), You'd have lots of space and spare rooms to do whatever with (unless you have big family, then it means you have enough room for everyone. Or maybe this mansion is big enough for both.)
Cons: You'd either have to spend a lot of time cleaning it or spend more money to have someone do it for you, people might be more likely to want to rob you cause you clearly have more money.
Those are just the first things that come to mind.
Pros: Lots of space, bragging rights, if you have a mansion you are probably wealthy so you have a lot of expensive and nice stuff.
Cons: Too much space so you could feel lonely, a lot more cleaning, wasting rooms that could be given to people in need, and you have a slightly higher chance of burglary so you would need to spend a lot on a security system.