Maths is probably easier to get an offer for a top uni (Cambridge, Oxford, Imperial, Warwick). However you would need to sit STEP to be accepted into Cambridge, Warwick and Imperial. Getting an Oxford interview also requires an exam. STEP is VERY hard and can make or break offers, for top maths unis, in my opinion, getting the offer is the easy part. Fulfilling the STEP part is the hard part.
Economics on the other hand has more applicants to places at top unis as far as i'm aware (Cambridge, LSE) so would make it harder to get an offer and they also give out less offers per place I think because more people are likely to hit the maximum economics offers of A*AA offer compared to COWI maths offers of A*A*A + STEP.
In the end, if your maths module grades are good, 95% + average will definitely do, you are probably more likely to get offers, however you need to be confident at STEP. Or you can go for economics, hope for an offer, and then be almost guaranteed a place if you get one.
Just to show how hard it is to hit maths offers for Cambridge, when I went up to the open day they said that they give out twice as many offers as they have places just because so many people will fail STEP. Warwick and Imperial are much easier on the STEP if you're not aiming for Oxbridge.
So in my opinion. Aiming for Oxbridge maths is harder for Cambridge, I don't know enough about the Oxford applicants to places to say. However for Warwick/Imperial/LSE maths is probably easier to get an offer. However if your grades are excellent and can almost be guaranteed an offer then go for economics. As someone above me said, you can do maths with economics. I know for a fact LSE run this course.