We fly into Orlando because we usually go to Vero for several nights first and then go to WDW (I know... just the opposite of what most folks do!!). But, if we were only going to Vero, we'd probably try to fly into West Palm since it's closer.
For us, the flights are not as frequent as to Orlando so that's also a consideration. The Melbourne airport is quite small and the schedule more limited, but again, if we were only going to Vero and the fare and schedule worked we'd do it. We had to make a detour to the Melbourne airport last year when we had problems with our National rental car -- airport was nice, not as hectic as Orlando.