As a long-time crocheter who made the leap, I definitely recommend learning to knit. It's such an interesting process, since the mechanics of knitting are so different (what with the straight needles, and all). Before I learned, I simply couldn't wrap my brain around how someone could draw up a loop with those pointy things. Just goes to show how limited human brains can be . . . even though I saw knitting all the time, and looked at hand-knit soakers, socks, clothing, and the rest, I still couldn't quite believe it was possible. We're such funny creatures.
Here are my own observations about the differences between knit and crochet. Your mileage may vary; this is in no way definitive or absolute.
Crochet is gorgeous, and still one of my favorite crafts. It makes beautiful lace and amazing decorative edges, adapts itself wonderfully to truly 3-dimensional designs, and is just plain fun. It's also handy to only have one or two live stitches on the hook at a given time, since you essentially bind each stitch off as you complete it.
Knitting is incredibly utilitarian, with the added bonus of beauty. It makes a more functional, durable fabric than crocheting does (less bulky and usually smaller holes). In my experieence, when crocheting is stretched and pulled, it's incredibly hard to block it back into shape, whereas knitting responds more forgivingly to actual use.
Assuming you used the same diameter tools and yarn, and made identically-sized swatches with knitting and crochet, knitting would take more stitches, more time, and less yarn than crochet.
YMMV, but those are just the things that I've found to be true in my knitting and crocheting.
I find that I knit a lot more than I crochet now. I've knit quite a few hats and socks for my family (four small kids plus DH), as well as a couple of soakers and other things. I have, ahem, more WIP's than I like to admit, but that's definitely a pattern to which I don't hold monopoly. ;o)
Learning how to knit opens up a lot more options in your ability to make things, where crochet just doesn't lend itself as well to some projects. For example, there are crocheted sock patterns out there, but stockinette stitch really does work a lot better than any crochet stitch for socks. Knowing both crafts is such a neat way to expand the way you see garment construction. It's a lot like being bilingual (which I was able to enjoy for a short time in college).
The other most notable part of learning to knit was how clumsy I felt. I'm a pretty coordinated woman, and I can catch on pretty quickly to most of the things I try. But knitting? I was cross-eyed with my tongue sticking out for nearly a full day at the start. Then my eyes slowly uncrossed; but it took another couple of days before my tongue could rest comfortably behind lips again. It was at least another week or two before I was decently fast. So, hang in there when you're learning. It's hard at first, but once your brain and your hands really start working together, it's like anything else you've learned to do in life (walking, talking, typing, stirring a pot of soup). It becomes automatic pretty darn quickly. Which reminds me, I've been meaning to do a post on how I learned to knit. . . .
So, to all you crocheters out there: go learn to knit. You'll be glad you did. :o)
And, to all you knitters: go learn to crochet. ;o)