TNG is very formal, everyone is rather stiff because they have to show proper respect to rank. On DS9 it’s much more casual and to me, a lot more fun.
I find this kind of funny, because my problem with TNG is that the cast (other than Picard) is too casual. In TOS, Kirk, Spock, and McCoy are best friends, but they’re also fellow officers, and they treat each other as such. The same goes for the lower ranks. TNG, it seems to me, did this a lot less.