Hi Six,
Thankfully I haven't seen too much of that corrosion trouble yet but I'm sure I'll get my share.
I guess it all depends on what type of powder and paint you compare. There are several different type of powders: Epoxy, polyester, nylons, vinyls, acrylics and hybrids, just like paints. Different powders for different uses (Indoor or outdoor, Marine, architechture, machinery, furniture etc.) otherwise there'd be only one type. Epoxy is very chemical resistant and very hard but performs poorly outside in the weather a polyester also has good properties against chemicals and would work better out side in the sun. Chose the right powder for the application and it should have a long serviceable life but.... nothing lasts forever including other types of coatings.
Corrosion under the coating is usually caused by poor or inadequate pretreatment. I would assume it would get started where the coating has not properly bonded to the surface. Maybe from some oily fingers or deposits left over from the last cleaning or uneven curing. You do your best to clean and prep the part before you coat it.
Clear coats add thickness, depth, specific gloss, abrasion protection for the under coat like metallics but not a hole lot more then that.
I still think a powder coated finish will far out last spray urethanes and enamles and even some epoxies.