Those are almost totally opposite from each other. It would depend on a few factors. Budget being the first, obviously CS is going to be less.
Do you spend a lot of time at MK? WL would win then because of it's proximity but other than going to MK, you pretty much are going to have to bus everywhere from there so transportation is about even. Yes, I know you could boat to MK then monorail from there to TTC then to Epcot but really, I think bus might win on that.
Both have some pretty good food choices, I haven't tried the new places at CS but they look pretty good.
What kind of room are you thinking of? With 4 adults, you could certainly fit in a regular room but the girls are going to have to share a bed and that's not a lot of bathroom space for 3 women.
I would take CS over Riverside but I love the Southwest theme and the fact that it's a convention hotel is actually a plus for me since you get more amenities. WL, SS and Riv. are the only deluxe resorts I haven't stayed at but I have visited WL many times. I've stayed at CS a few times. The rooms are going to be bigger at WL so with 4 adults that is going to help.
So, it all depends on your wants and needs and budget. All things being equal, even with budget aside, I'd probably choose CS but then, I'm not a big MK fan.