To be honest, I woud use both. If the greenhouse is for general use, you'll probably want an area with overhead irrigation and someplace a bit drier. That dry space can be well served by drip or spot spitters.

for that size, I would weight a little more on overhead, probably misters or very fine spray. in my own (6'x8') I have both. overhead misters that do full coverage, and then a line of drip. it's all on my automatic timer, but both have manual shutoffs so I can run just one, or the other, or both.