Too hard to control water to cream ratio, this is where the mug, bowl, scuttle, whatever always wins. I used to do it this way but I found that most of the water I had loaded into the brush just ended up running down my chest.
I build mine in a bowl. I tried doing it on the old face and didn't have much luck. I'm not sure one way is worse/better than the other (aka matter of personal preference), but other people might have different opinions on it.
For me, with a good soap, building lather on the face is fastest and works most effectively. I find that building lather in a bowl reduces the effectiveness of the brush because, once built, there is less pure soap left in the brush. It's sort of a "scooping" action to get more lather from the bowl, rather than a re-lathering. If built directly on the face, there's always a bit more pure soap in the brush that's ready to build another pass-worth of lather.
Creams, on the other hand... I just don't understand creams yet