If we're being honest, i don't think it really matters which option you choose. I did double science, purely because i used to be so very sceptical, and had no interest in it (thats changed now. Now i'd like to do it at degree level!) I mean, if i had the choice again, i would have chosen seperate sciences, but mainly because of the interest factor.
I guess it probably goes into a similar depth of information as double science, but at the end of the day I dont think it will affect you're A level performance one bit. Information you might not know as a result of doing double science would probably be either irrelevant, or easily and quickly taken up as part of private sudy by seeing it included in a similar topic in your AS textbook.