Advanced highers are at a higher level than A-Levels full stop. As far as Im aware, the level of difficulty goes something like
(Least Hard)
Standard Grades
GCSE's
Highers
A Levels
Advanced Highers
(Most hard)
There's not too much difference between advanced and A-Levels but the difference being that Advanced highers are done in 1 year, whereas A-Levels can be split into 2 and the level of knowledge isnt as high. Thats why in the first year of uni doing pharmacy, I found most of the work quite easy, having done advanced maths biology and chemistry, whereas the english students and maybe some ROI students found it harder because they'd not had as high a level of knowledge required for their exams.