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…
Make sure your shader is scaling the raw values properly. Multiply your colors by 2 then subtract 1 will move them from 0-1 to -1 - +1. So painting with 127 (or 0.5) will be 0 offset. 0 will be -1 offset and 1 will be +1 (or subtract 1 and multiply by 2. A constant bias scale node is what you need. Bias of -1 scale of 2.)
Training is focused solely on animation -- all characters are provided rigged/skinned. Subtract 10000 and you'll have the correct amount for 18months of training.
It's not just subtractive - it also supports unions and intersections. The difference is that it's all live. Move things around in real time and watch the geometry dance. :)
@gnoop it looks like when subtracting, they never go beyond an existing edge. Is there a reason for that? Here is Smart Extrude in conjunction with working pivot.