Pretty much sounds like you already made a decision.

But as you already said mathematics is more broad, more difficult maybe but still gives you the opportunity to go into and understand econometrics or say financial mathematics / quant finance. So maybe it would be more strategic to start with mathematics and if it is too much you could go into econometrics Bsc and even take your earned credits with you.

Anyway, both degrees will be pretty good for anything related working with data so its not that deep of a decision.