[BSousa]

I've worked with Unreal Engine 3 for a few years in various companies and in all of them we modified the source code to better suit our needs and never really had much of a problem merging new releases.

As for Unity, I like what they have done, but they are falling behind recently. They are VERY slow to fix bugs and they are late on releasing needed and promised features (GUI system took them what? 3 years from announcement to release?). Also, retina macs are what, almost 3 years old and there is still no support for that (I don't know how unity works with Windows Hi-Dpi support, anyone wants to chime in?)