I have both. The Fiskars has a head that is shaped more like a wedge. It is much better at splitting wood than the Estwing, because from the time the edge hits the wood, it opens up pretty quickly. If you think about it in terms of knife geometry, it is kind of like a knife that has a sharp edge, but then is thick behind the edge.
The Estwing on the other hand, is much more like a hollow ground knife. You have the edge, and then it stays pretty narrow until you get to the poll. It is a much better cutter, but I find that for chopping it gets stuck in the wood much more easily than Fiskars, and falls way behind when we talk about splitting. But when you need to press an axe into a cutting role, it outperforms the Fiskars.