As has been said, an unmodded NES top loader is not worth the price unless you REALLY want it on your shelf. Coaxial cable only, AND no game genie support, as game genie is too big for it. Game genie isnt really a deal breaker for me, but it is annoying.
I recently found one for a pretty good price and I am glad I have it, but I never use it. Plus I am not the biggest fan of the dogbone NES controller.
As for the SNES mini, I have one and have not noticed any difference in picture quality to a regular SNES. That said, they are hooked up to an older (pre 2000) TV, so that may not be the best judge of picture or sound.
As for why they cost more, because they are less common so people assume they should be more $$$. Some stores near me charge $5 more, but that's it. There are rumored to be mini SNES controllers that originally came with it, but I cannot confirm this. I'm not the biggest fan of the power and resent switch. They feel tight and tend to stick, plus no power light.
Either way, unless you are a die hard collector who wants them on your shelf, stick with the originals.