Definitely do specialist if you enjoy maths
For one, it will help you in methods immensely. Calculus and trig, which probably make up around 60-70% of the methods course just becomes so much easier when you do spesh
Your maths in general will just be at a higher level when you do spesh, and your level of understanding and applying will be so much better and that also helps for methods and can even help a bit for stuff like chem and biol - which both have a lot of application type questions
Plus, the scaling for spesh for the following years is predicted to be 11-13
So if you get a 30 in spesh (which is around 60% at the most on both exams) it will go to at least 41, while a 43 in further would get scaled down 1 or 2
Also, if you're a good math student, further will be so boring and you will not be motivated to study like you will in spesh, and as a result you won't do as well as you could have done. You'll also feel as if you won't need to study for further, so definitely choose spesh