Which degree is better? Watch
Both courses will usually contain a substantial mathematical content (or *can* if you chose mathematical options) - it's up to you to decide if the level and type of maths content in a single honours maths degree is what you're looking for or whether you'd want to enhance your maths skills/knowledge in non-econ areas.
If you do decide to go the maths & econ route then do your research. Combined honours degrees can often be poorly organised and poorly supported. Ask about degree structure, timetabling, how your final year project/dissertation is managed. Ask which department "owns" the course and what sort of communication between departments takes place. Ask if you'll be taking second and third year econ modules where academic staff assume all the students have taken a full suite of econ modules in first year or whether all modules cater for students taking combined honours.....and ask to speak to current students about how they find the course and whether they feel they benefit from the combined honours or not.
Some universities absolutely nail combined honours courses and the students have a great experience and support....in other places you could be signing up for a sub-standard experience where you'll be marked by both departments assuming you're taking a single honours degree in those subjects...in other words you'll need to work twice as hard to do as well as single honours peers. Be 100% sure you know what you're signing up for before you commit to it.