A small cohort could be a good thing or a bad thing. There's less luck involved in having a large cohort - it means that if you have a bad day on one of your sacs or your exam it's probably not going to affect your study score quite as much because you'll have people of a similar ability to you. For example, if you're in a small cohort and are rank 1 but are doing far better than everyone else and then have a bad day on the exam, your ss is going to take quite a hit, but if you're in a large cohort and have people doing almost as good as you, you'll still get a decent mark for GA1 and GA2 because of others exam scores.
It really just depends on how your cohort is though - if you're in a small cohort but have others who are scoring similarly to you then it won't be much different to being in a large cohort.
You'll probably get more attention from your teacher given they don't have as many students to deal with, but if they're not a good teacher you won't have any other teachers for that subject to ask.
There's positives and negatives, so it's not really possible to say which is better. Your rank is definitely more important than if you were in a larger cohort though.