Well, Epiphany was designed to be "simpler" than Galeon. Personally, I like options, so I like Galeon Better.
Both look and function very similar, but Galeon is more customizable.
Also, In my experience, both aren't very stable, thats why I use Mozilla Firebird.
Epiphany wasn't made the official browser because its better, but because the GNOME team had better communication with them.