They are two very different things. I own both, they both (BBK28 sellier and an old black 226 Chanel), and the use I have of them is very different.
They both are bags that go from day to night.
Chanel is more always the same because it is identifiable, hermès is a jolly that maybe makes me look more like an old lady, wich I am not, but it really goes with everything and has a versatile non tiering aspect that Chanel doesn't have. That said I am happy to have the Chanels for a change from time to time.
It also depends on how much you want to fit in the bag and how formal you want your bag to be.
For very formal occasions I would more go for a clutch or a smaller version of kelly or of Chanel.
Honestly? I would make this kind of long term plan if you want to be covered for all kind of formal occasions:
First purchase BBK, sellier, 28.
Second, Chanel 226 or 225 reissue flap in either black or blue.
Third, a Kelly Pochette in black, PHW.
If you want to skip one of these three I would go for a Chanel, and then after that the Kelly Pochette.
But that's just me... you have to really think what suits you more and what you like more.
Sorry to make things more complicated and more expensive.