Poncho vs umbrella
Many guests will opt for a poncho as rain gear, either packing it themselves or springing for a Disney-themed one in the parks. It's easy to see why as they can be effective cover and, once the rain is gone, you can roll it up into your bag. However, it still can get pretty wet, not to mention more than one guest suffering from getting a poncho caught and ripped while on a ride.
An umbrella may be a bit bulkier and harder to handle when getting on a ride. However, it can also offer better rain protection, especially the larger ones. A couple of big ones can protect multiple people at once and might be preferable. It's up to the guest as each has their strengths and a question of how wet they'd want to get.
Packing for rainy weather
Whether you go for a poncho or umbrella, there are a few other things needed for a rainy day. A couple of towels can be nice, not only for your personal items but wiping off seats and other spots.
Another must are good shoes; waterproof is the best option but make sure they can handle rainy weather. Also, an extra pair of socks or two go a long way to easing your comfort in case your first pair are soaked through by lunchtime. Ziploc bags are also recommended for ponchos or other clothing that gets wet. On top of all that, a water-resistant bag to hold everything is another need to ensure items like phones and such don't get too soaked.