If I get comments, I like both, short and long ones.
When I'm commenting I'm sometimes overwhelmed by the story and don't know how to express myself, sometimes I just pick a sentence or paragraph and quote it to highlight my opinion.
I get e-mails of my AO3 comments and it was nice browsing through them and remembering the fics they were about for my recs-post for day 14 snowflake-entry.