It really is a bit of both. Mostly - I am looking at the topic. If it is a topic that I am really interested in, the length doesn't matter, just the quality.
However, that being said, I will admit that I sometimes click out of a hub if I find it is too long. This is because I am trying to read it on my spare time and if the hub is just too long... I get distracted and don't really grasp what the writer may have intended for me.
Too long of hubs can be unfair for both the reader and the writer. The Reader because maybe they are just looking to read something quick on a short break (or in between there kids hounding them) and the writer because if the hub is too long - readers like me might be intimidated and back out or get distracted in the middle. It might now get my whole attention as I have other things going on as I cruise through hubpages!