Okay, so this is just my opinion and I feel like other people who have had different experiences will feel differently, but I don't do either.
I really don't like watermarks. I know when I see art with a watermark over it ALL I see is the watermark. It's really distracting to me and makes it so I can't focus on the art. I guess it depends on the size and placement, but since people want to use the watermark to keep their art from being stolen they usually make it big and prominent so it can't be cropped out. Also, as someone who likes putting together blogs and pinterest boards of illustrations, I don't share stuff that has watermarks on it because I believe it disrupts the flow of things of the viewer (and myself). Credit and links in the text look so much neater to me.
Signing stuff isn't as intrusive, so I don't mind it. I just personally don't do it. Everywhere I post stuff online is linked to a profile, so people can tell who made the art.
That being said, I've had very limited experience with art theft, but in the few cases where it has happened other people have seen it and told me so I could ask the people who were posting without permission to take it down.