Well the jump between Further-Methods is a lot bigger than the jump between Methods-Spesh

Heres a little diagram of the difficulties of the three maths

[---||--------------------||----------||----]

Further-----------Methods--Spesh

If you were even average at Methods, you could do pretty well in Further (coming from personal experience)

Further is mostly statistics and information you learnt in earlier years, if you REALLY tried, then high 40's wouldn't be too hard to achieve but due to the simplicity of the exam, even one mark lost will most likely scale your mark down (you will pretty much need a perfect score to not lose any marks from scaling)

On the other hand, Methods will scale up a few points and gives you more opportunities and choices in your university courses (Quite a few courses require methods as a prerequisite). As long as you arent really struggling with methods, I believe Methods would be the way to go. With lots of hard work and use of resources (merspi etc.) it isn't too hard to improve. As with all maths, practice and more practice is the way to go.

I myself do both, so that is also a possible pathway you could take. This way you can always drop one if one of the either doesnt suit you.