On the Wii, all the VC games I played so far (NES, TG-16) run extremely faithful to the original. Heck, Nintendo even included the PAL gameplay+music slowdown in games which were affected, e.g. Castlevania 1, if you live in Europe, since they just used the crusty old PAL roms there instead of NTSC ones. Thankfully any title non-native to EU still runs at 60Hz on the PAL region VC, like all Turbo ones. Later PAL games often had less-to-now slowdown.
I play them on a 4:3 aspect ratio CRT, where there's no stretching involved and the pixels are sharp as a butcher's knife.
I know the original Wii had some scaling issues when set to 16:9 and playing Konami's Rebirth WiiWare titles, where the screen got blurry. Setting the console to 4:3 in the system's menu was mandatory.
I think that Mini NES might be worth even just for those seemingly Wii-Mote compatible controllers, if the price is right.