Grammar or Comprehensive school?

Hi guys. I went to a comprehensive school which is in the 300s for A level results 2015 (http://www.telegraph.co.uk/education...e-schools.html. I got into a grammar school sixth form which is in the top 20 in the country. However being there for a week, the teaching isn't amazing and I cannot do all the subjects I want.

I want to do economics at a top 10 uni (preferably LSE) and at the comprehensive I can do Maths, Further Maths, Economics and Geography (or Chemistry) and if I cannot handle A2 FM, I can just do the AS in year 12. However at the grammar I can do Maths, Economics, Chemistry and Politics whilst only doing further maths as an AS in year 13 (1 lesson per a week).

I am wondering which option should I choose as I can go back to my old comprehensive school.