I don't have a coverall to compare, but I've heard that the differences are that the coverall doesn't have flaps for holding an insert, and the Capri (size 2) extends larger than the coverall.
We have 4 Capris. I LOVE them. We use them for tri-folding prefolds or when DS is in a prefold. He's a toddler and on the large side. He fits the Capris better than Flip covers or Thirsties Size 2. He's got room left to grow, which is great.
Into+the+Kitty:They are both two sized. Ihave the size two of both. They fit pretty much the same, but the Capri has flaps for putting inserts into. I like the Capri better because I'd specifically gotten it to lay inserts into.
This. I bought two of each of the size one for prefold covers. If you're trifolding prefolds to just lay in the cover, Capris are better. If you're using them over a snappi'd prefold, I'd say the coverall is a better use of your money.