Hard to choose, but I think I'd say fairies. I don't go for the Disney style girly fairy though-I like Celtic style ones, or faeries, as they are often called. Far more believable and interesting characters in my opinion...

Fairies are in the warm sun and flowers, and mermaids are in the dark, cold water. I like fairies more (besides when I swim and pretend that I am a mermaid).

both

I like both fairies and mermaids though