I'm going to go with the original model due to the eject button and S-video support. I've never owned or seen in person the new model, but it looks pretty bland in pictures, even moreso than the older model. Though it wasn't susceptible to the mysterious "yellowing plastic" which plagued the older model and cartridges, which I s'pose is a plus.
The Original SNES for me. While I like the size, shape and design of the mini, the lack of S-Video output sucks. Although, to be fair, it doesn't make that big of a difference...
It depends on the game. Some of the later ones, that have a larger colour palette, have better-defined colours and more smooth blending. I guess it isn't a big difference, but it is noticeable, at least to me. Now, if I pop in something like Super Mario World or Zelda, it looks the same.