When using descriptive terms, should we say "fat" or "obese"?

http://www.thatsfit.com/2010/08/02/fat-vs-obese-whats-in-a-name/?icid=main|main|dl10|link5|http%3A%2F%2Fwww.thatsfit.com%2F2010%2F08%2F02%2Ffat-vs-obese-whats-in-a-name%2F
This little advertisement for an internet site poses an interesting question.
It gets down to this – which word, fat, or obese, might be more effective in giving such folks a wake up call? I think fat is pretty descriptive, whether or not it is motivational. What do you think?