Just going to pop in with my experience with this - please dont take my word for it though - this is what I just did.
So I actually did 3/4 Methods and 1/2 Spec in Year 11, and now I'm doing 3/4 Spec. Personally, I was glad that I did Methods earlier as I really would've struggled for time if I did two maths subjects in one year. That was the biggest advantage for me doing them separately. It is also easy as there were some concepts that people dong 3/4 methods already had a basic understanding of, and were able to go straight into the hard stuff.
That being said, theres a lot of people doing both at the same time, and its working really well for them. Spec requires a great level of detail and understanding, and I personally find that a lot of the tehniques you learn in spec can help make Methods seem like a breeze. However this really, really depends on your learning strategy. Its not the content that helps so much, rather about how you learn how to approach questions if that makes sense.
I think just as long as you do what you think is best for you, you will be fine. Theres no wrong decision! Good luck.