I love the leather sets in my car; they wipe clean, unlike cloth seats. When my son was a baby, he did stain some spots with his milk sippy cup, but the spots came out last year when I had my car professionally detailed. The seats are easier to brush crumbs off of, too.
I'm going with leather on this one. By the time your child is "sitting" on the actual leather seats they are much less likely to have accidents (all the freaking time). When their little is when the accidents are frequent. Sooo....leather when their young and cloth when they graduate to less of a mess.
my kids are 12 and 9 and with soccer they still seem to make the car a mess all the time. We travel so much, some weeks we are in our car more than our house it seems. I am really leaning towards leather being better.