The first one definitely sounds better! Further maths seems like the only pro of the second sixth form - can you take it outside of school if it's that important?
In all honesty where you go isn't always a big deal. If you put the work in then you will get good grades regardless. I do think taking further maths will benefit you a lot seeing as you will be doing maths as a degree.
Before you make a decision, email some of the universities and ask them how much further maths will improve you chances of getting in.
Doing a maths related degree such as physics economics or even maths itself without further maths is shooting yourself in the foot it will put you at a huge disadvantage