Like many people have stated, additional further maths is insane. It forces you to complete the latter mechanics modules. I don't have any experience with it, but I've heard that M5 specifically is known to be the hardest thing one can do in life.
I've dabbled into maths, further maths, physics, economics in chemistry (dropped chemistry for further maths).
1.) Chemistry (I dropped it because a: I didn't find it interesting and b: I subsequently found it difficult)
2.) Physics (some of signalling was pretty difficult, but overall it's not too bad)
3.) Further maths (interesting content which overrules any potential difficulty)
4.) Maths (combinatorics can die in a fire, but the rest is fine)
5.) Economics (only slightly more difficult than GCSE business studies, which I suspect will change soon)