I think this is one where different people have different opinions.
I can see points for each one; our personal favorite park for rain is actually Epcot.
Even though it can be quite a distance between things, there are a lot more places where you can go in and spend a lot of time on your own schedule (like the shops in WS, The Land, Imagination, Living Seas, the shops when you leave rides, etc.)
At MK, most of the queues are covered, but the rides are short and once you are finished riding, you mostly have to go out in the rain to get to the next place (although they aer close). MGM has mostly covered queues and long shows (so you are in out of the rain for a long time). But, for a lot of shows, you will be leaving a theater at the same time with a theater full of people - all looking to stay out of the rain.
At Epcot, for the things that you do on your own schedule, you can leave when the rain is lighter. For MGM and MK, your show might be released in pouring rain - you won't know until you get out.
Where ever you go, have a plan of where you are going next and keep the map handy so you know how to get there quickly as you can.
As has been pointed out, you are more likely to run into light rain or short periods of heavy rain than a whole day of heavy rain.
One thing to be aware of with a wheelchair if you haven't used a rain cover much. Look out for puddles - if it's raining hard, puddles form rapidly and some are quite deep. You might get some splash from the wheels on the bottom of the inside of the rain cover. Not a big problem, but something to be aware of while you are taking the rain cover off. (I found out because I couldn't figure out how DD got wet when I was being careful to keep the outside of the poncho away from her when I removed it from her wheelchair.)
Also, we carried a plastic bag to fold the poncho for the time in between rain showers.