Personally, I prefer reading completed fics because I can binge read them lol. But if a fic in incomplete and it REALLY catches my attention I don't mind. Also with WIPs there's always a part of me that's concerned the fic will be left unfinished and abandoned.
I tend to prefer completed fics, mostly because I read oneshots and well - they’re either completed or not published uwu If I really like the fic, I don’t mind reading WIP but I have a habit of forgetting to read when they get updated because - well, it has to fit into my schedule. ^^
I like reading both, especially if I like them a lot. I leave comments too to give them love and support. Recently for a different fandom on AO3, I was commenting on this story that had zero comments and the writer answered saying she cried of joy seeing my comments. It made me happy that I was able to do that. Completed and not completed stories have the same potential for a masterpiece so yes I read both :)
it depends if the first few chapters are good, i'd totally give it a go while making sure i comment at every chapter so i could motivate the writer to continue writing till they can end the story! completed fics are ofc better ;")