Scrubbing Bubbles wins.
I just washed the belly of my 360 this weekend for the first time in ~75hrs. Ive been using XTreme Clean that I bought at OshKosh. It works OK. Ive used Simple Green. It doesnt work any better that the Turtle Wax car soap I use on the rest of the plane. The Scrubbing Bubbles took 90% of the soot off the belly on the first pass with a paper towel. I was amazed. The thick stuff near the pipes took 3-4 applications but I hadn't cleaned that completely off in at least 150hrs.