I have a few cards of different grades, and tbh I can't tell the difference whether I'm using an expensive one of a cheap one when I'm taking pictures. Maybe if you are battering the motordrive you gain a little on the buffer, but most of the time I think there is a bit of the old Emporer's New Clothes going on.
There is a little bit longer to wait downloading them to the 'puter, but not anything that bothers me too much.
So I'd say just buy the cheaper ones and see how you get on.