I think both are equally important. Some lines, specifically Matrix Amplify and Sleek Look are made to work together as a system. You won't get the result you're supposed to get at all if you don't use both together.
And as for cheap shampoo being just as good, I'll be the first to tell you I know next to nothing about drugstore brands, but I do a lot of extensions, and before that hair goes in someone's head, it's smooth, silky, perfect, like virgin hair. The women who don't heat style the crap out of it, and use good products, maintain that look. The one woman I had that refused to give up her herbal essences (no joke, 1200 for hair, why would you use $3 shampoo on it?!), that hair looked like it had been chewed on by a goat.
My opinion? It's worth the extra cash.