I think most people want to lose weight for both reasons but which one of these is the one that motivates you the most?
For me, I'd say that it's mostly about health, but of course I also would like the side effect of looking good too. A lot of people see what they look like and are unhappy and that is the motivation.
I think when we are completely honest with ourselves with our reasons and feel a strong enough resolve, then we have more self control to get to where we want to be.
Just some thoughts...