You can read some end-user comparison of the two backends in one of my previous posts.
One another great thing is the speed with which the GTK port is advancing now. Acording to one of the webkit and epiphany developers, Gustavo Noronha, WebKit/GTK might be the default backend for Epiphany in Gnome 2.28. That's Fedora 12! Whoa! I am already restless :-) More specifically he's asking us, the end users, to
"come help us (the epiphany devs) test and get a regressionless Epiphany with WebKit/GTK+ for GNOME 2.28"Well, this might be worth a feature page for F12 ;-) And even though I am using Midori to test WebKit (I use epiphany as my primary web browser and it's rather inconvenient to keep two versions of epiphany installed; and I actually find Midori very interesting and light-weight) I think I will resume testing WebKit in Epiphany as soon as Fedora 12 development starts.