A canoe could carry a larger load, but who wants to store a 13 foot long canoe in their garage as a "just in case"? I'd rather store a couple square feet of deflated raft. A four-person Walmart raft will only set you back $40 and weighs 10-15 lbs. The quality is horrible, and I'd never want to take one on any serious water, but for emergency use it seems like the perfect combination of ease of use, carrying capacity and price.

If you're already storing a canoe though, it would make sense to just keep what you have and are familiar with.