I remember both of these! The 280s have been around quite a while, have a bigger, more comfortable earpiece, a bit more neutral sounding than the 518s. For me they're the better choice.
518s are not comfortable, they have a slight 'bump' in the bass, and they smooth out the highs too much for transcribing imho
However I liked them for listening to older recordings like (Sinatra, 60's rock) that were a bit bright sounding or poorly mastered, stuff put on CD in the 80's, etc.
But listening is so personal. Best if you can do an audition somehow with your own music.