I would have to say.... Elton John, because i think whereas Billy Joel might have more skill in playing certain things, Elton John somehow makes whatever he plays sound a lot better.

There's your answer...
There's your reason 

Of those 2 pianists I prefer Billy Joel. Both write great timeless classics, but there are way more songs by Billy Joel that really suck me in to their theme/storyline. Maybe I relate to Billy's lyrics more because I grew up on Long Island nearby where he grew up, or maybe I'm just generally attracted to darker lyrics. Even Billy's happier-sounding songs have some cynical or wise-ass undertones. The lyrics of Captain Jack have haunted me for years. I was coincidentally playing that song tonight.

As for the piano playing of Billy Joel compared to Elton John, Billy is way better. Songs like "Scenes From An Italian Restaurant", "Prelude / Angry Young Man", and "Root Beer Rag" aren't easy to play, but after learning tons of his songs I have to say "Stiletto" is the most difficult. If you never heard "Stiletto", check it out.