I've seen both used for かなしみ several times; what's the difference in nuance between the two? If I had to make a guess I'd say 哀 seems a little more archaic or poetic? Or more towards the meaning of 'grief' than 'sadness'? I'm probably way off the mark...
From what I can make out of it, 哀しい seems to signify more than just simple sadness, and closer to the lines of 'heart-wrenching', so 'grief' might not be too far off the mark. 哀しい appears to be more complicated, and stems from multiple sources, rather than just sadness as 悲しい is used for.
@ Yzlot - That's what I thought... it's hard to put into words, but 哀 seemed more... profound than just 悲, in the way you might say you're sad you've lost your favourite pencil vs. a more complex emotion in a deeper situation?
@ Toritoribe - I see! I wasn't aware of that. I've actually seen it used in songs that use an older style of Japanese or aim for a very poetic feel in the lyrics, so I think that's where I got that impression. That and 哀れ stems from the Heian period, I think? I remember coming it across at some point while reading The Tale of the Heike...