The difference between the Sandisk 'Wii-branded' ones and a generic SD card? Probably minor at best. Both will work fine on your system. It's possible the generics will have a slower data-transfer speed, but even if so that's not a big deal; you're using it as a memory card equivalent, not for 'streaming data.'
Save yourself some money, go with a generic SD card. They work fine in most cases. You're mostly buying the 'brand name' with the Sandisk Wii SD card.
I have not been able to find the read and write speeds for that card. I figure it's an average 10mb/s card, but it has a Wii logo on it and sells for more
My card was $45 without shipping, but it was 2GB, 133x (20mb/s, the fastest SD card you can get, and the Wii supports it). Speed probably won't make a HUGE difference, but without a doubt, a faster card will mean faster save and load times when using the card, and much faster times when transferring things from your SD to the Wii flash memory.
If you don't download VC games, you would probably be fine for a very long time without an SD card at all.