Bals are comfortable almost the second you take them out of the box. With Chanel, it's hit or miss, depending on the style and material.
Bal sizes are more consitent than Chanel's. Let's just say that I'm pretty comfortable ordering Bal flats online, but would never do so with Chanels because I've seen large size fluctuations in the Chanel classic ballerines of the same style/color/year. I always always always have to try them on.
But, hey, I like them both enough to have multiples of both styles - classic colors from Chanel (black, light beige, dark beige, brown, white) and bright colors from Bal (see my avatar).
My suggestion? If at all possible, go to the store and try them on, see what feels and looks better on your feet then decide.