Some people tend to stereotype knitting and/or crochet! And I hate it when they do that!!! They will say that knitting is better for clothes, and crochet is better for doilies and blankets, but I completely disagree. Crochet! Magazine has some fabulous crocheted clothes, and knit doilies and blankets are equally as beautiful as crocheted doilies and blankets.
So why does it have to be either/or? As long as you're aware that crocheted stitches make a slightly thicker fabric than knit stitches do, then you can make either (or even both) work for whatever project you want. Really, it depends on how you want your final object to look.
I recommend flipping through knit and crochet stitch books (such as the ones put out by Interweave Press) to help you get an idea of the amazing range of patterns for both crafts. If you're strictly a knitter, you might be pleasantly surprised at all the things you can do with crochet, and if you're strictly a crocheter, well, the same thing applies to knitting!!
So it's not either/or; it's both/and!!