I would think a rain coat would be more of a hassle than a poncho, and likely take up more space.
First trip, we used Dollar Store ponchos. They were sleeveless and if you have hair, they won't stay on your head. Sure they were easy to take on and off and we were fine throwing them out when we were done, but they don't keep you dry.
Second trip, we used Coleman ponchos we found at Big Lots for $5 each. Much better quality, kept us dry, and we just hassled with them for the week and didn't lug them home.
Last trip, we brought similar ponchos to the Coleman ones but had left them in the room when it started to downpour while we were at MK. We bought the Disney ones when we needed them, and surprisingly they were inexpensive and good quality. I think next trip we will wing it and if it starts to rain, we'll buy another Disney one when/if we need it.