Mathematics vs Mathematics with economicsWatch
Have a look at the course structures. If you're particularly interested in financial mathematics then see if Imperial's course has modules pertaining to this. Also, make sure you know the full structure of Mathematics with Economics, as there may be some optional modules in the Mathematics course that interest you which aren't available if you're doing a minor subject alongside it. Have you gone to offer-holder open days? Those would be helpful if you can't decide. You might also want to consider what kind of environment you're looking for, as going to uni is London will be quite different to Warwick.