I know right, 58 votes as of this post and split exactly 50/50.
For me, while I dearly love them both, I had to vote for DKC. It's atmosphere, soundtrack, and pacing make it the better game to me. Yoshi's Island is still an awesome game though.
Always loved this track in DK2.
The way they managed to create these instrumental tracks in only 64kb of Ram is a testament to not only Sony's engineering with the Audio, but RARE as developers.
Visually, Donkey Kong's Pre-Rendered CGI art has aged, but it hasn't impacted it's gameplay at all... And it still has a good charm to it.
Yoshi's art however is still fairly timeless, mostly thanks to it's art style and it's use of the FX chip for things like scaling of sprites.
Both are awesome games in their own right and is a testament to how amazing the Super Nintendo really was from all angles... It's no surprising that they are tied in the poll.