Both crafts use your hands, and learning with your fingers as well as your eyes greatly benefits you in both, but crochet is, overall, easier to learn as a first yarn-based craft than knitting is.
Crochet only works with one stitch at a time and therefore progresses in a snake-like pattern, where every other row is in the same direction. This means that it unravels in a zig-zag pattern, too, so if you drop the loop off your hook and it starts unraveling, you lose less work, but you also lose all the work till a mistake if you go back to fix that mistake. Also, working one stitch at a time makes last-minute adjustments a lot easier to work into a fabric. Some crochet items can be done freestyle, without a pattern or counting.
Knit has all the stitches across a fabric on a needle at once. This means the fabric develops vertically, and unravels vertically, too. If you drop one stitch in the middle of a fabric, you will develop a loose strand across the fabric where the stitch has vertically unraveled, but the strands on either side will be safe. This is repairable once you understand what knitting is supposed to look like, and you can purposefully unravel some stitches vertically to reach the stitches involved in a mistake, and repair it, with less work lost than with crochet. However, if stitches fall off of your needles, you lose more work, too, for without anything holding the loops, all of the stitches will begin unravelling vertically. Having all the stitches involved on the needle at once means that last-minute adjustments are extremely difficult without problems; to work properly, any adjustments' locations have to be deduced mathematically. Any shaped knit items involve counting rows and mathematically placing adjustments.
A hook grabs the string to crochet. The hook can be used somewhat imprecisely; as long as it catches the yarn, it is fine. The beginner can learn how different yarns work with as much ease as possible.
To knit, you must learn to catch the string between two pointed tips. This requires precision and hand-eye coordination (but not necessarily depth perception, because you can feel the distance with your fingers). Each yarn requires its own precision, so the beginner must learn both how to be as precise as is required and how each type of yarn works.
There are several stitches in crochet, but all are related. Once a beginner can crochet the slip stitch and the double (European treble) stitch, he is able to crochet all of the stitches. The main difference amongst the stitches is how many times you wrap the yarn around the hook before working into the fabric. Crochet stitches can only work in one direction in a fabric, not both directions in the same fabric.
Knit has two main stitches that are related, but it is difficult to see that. Each stitch is actually the reverse of the other, so to learn them both the knitter must learn a completely different way of placing the needles, wrapping the yarn, et cetera, for both stitches. However, this reverse aspect of the stitches means that is it very possible to knit backwards, though it is difficult. (The purl stitch, in particular, is difficult to learn backwards.)
Also, because of how the fabrics work, crochet fabric tends to be somewhat bulky and often full of holes when compared to knit fabric, but crochet fabric is much easier to shape. Making, for example, a teddy bear is moderately difficult with crochet, but it is a nightmare with knit. On the flip side of that, a knit bathing suit might be nice, but a crochet one would probably reveal enough to cause trouble.