Depends on the amount of rain. If it is just a light rain then you should be fine and with a lower crowd it should be pretty nice (sort of like an on/off rain that isn't heavy). An overcast day is no problem. However, if it is a constant downpour (rarely is there from my experience) then it would be sort of awful (annoyingly, this happened this past December, waited for Fantasmic in the rain, they kept saying it wasn't canceled, then at the last minute it was canceled and it rained all night). I also did get a really rainy day in January before, which for whatever reason wasn't a huge deal then and ended up going on a lot of rides.
I usually prefer less of a wait, so as long as it's not crazy rain, I have little issue with it. Especially Magic Kingdom, which is so packed together that there's always buildings to walk in. EPCOT also isn't so bad since you can usually find places to hide out in that are pretty big, and the whole World Showcase.
But it's really up to you and how intense the rain is. Light rain = good. Heavy rain = not so good.
Just looked, it seems like it may be a bit of a heavier rain tomorrow. Could still be worth it. I'm not sure if there have been other rainy days around there lately, so maybe you can base it off that. But with the weather I'd say Magic Kingdom is the way to go.
