Well, I can look to the likes of my 19 year old powder coated Bontrager, the life and the miles under it and compare it to some of the (expensive) enamels etc on some of my other bikes and I know which one I would say is more resillient to a life off road and a British climate.
If you wan't anything fancy or a particular colour then paint is the way to go, but if it's a single colour or a simple fade (which a few shops now seem to do in powder) then I'd pick powder every time.