In my personal experience, different qualities (and read speeds) of SD Cards do make a difference, not only to load stuff, like homebrew apps on The Homebrew Channel (my previous SD card with a read speed "4" took longer each time I turned my console on than my actual one, which is a "10"), but it can also make a difference with things like emulation, not only by reading ROMS and loading them faster, but actually also while playing some games, like those on CD-based systems/addons.
For example, I had a problem with Lunar : Eternal Blue on Sega CD with my prior card, and the game would constantly get small hiccups, but with my current one, faster, that problem doesn't exist and plays smoothly.
So yeah, I would advice anyone to get a good SD card instead of a cheap one, specially now that prices for 16GB and so cards have been dropping. Hell, you can get a good 32GB SD card for less than what a mediocre hamburger on a Burger King costs, at least where I live, and that size should suffice for pretty much anything I can imagine using my Wii.
As far as brands, my Wii never had a problem with Kingston SD cards, so I usually go for those, but I'm sure other good brands will be fine too.
