They both use BSP, Source uses additive BSP, Unreal3 currently support both additive and subtractive BSP. Unreal2004 was subtractive only. From what I remember most of the Unreal3 documentation suggests using additive. Although, as far as I know, most people don't use BSP much anymore for anything other than the initial…
Monkey - Yes I actually plan to start learning the Unreal Engine soon. I own both Gears of War and Unreal 3 so no worries there... I'm just a little intimidated about learning a completely different editor especially considering that Unreal uses subtraction whereas Source uses BSP. I'm working on a new mp project with…