Illuminations isn't until 9:30 on CP nights. You'll be fine.
Pros and Cons for all of them.
5:00 show is not quite dark when it starts, but is before it's over, so you miss some of the "candlelight" ambiance. However, you get to see and hear the show without having listened to the previous show while waiting in line. Can't linger in the theater afterwards as they have to get it ready for next show. Plenty of time to eat dinner and find a spot for Illuminations.
6:45 show is completely dark, but you've listened to at least part of the show while waiting in line and you have to get out of the theater so they can get it ready for the last show. Plenty of time to get a quick bite and find a spot for Illuminations.
8:15 show is completely dark, but you've listened to at least part of the show while waiting in line. Once show is over, you have short time to wait for Illuminations which is nice, but you probably won't be on the rail anywhere around the lagoon. That's not a big con as Illuminations is easy to see from just about anywhere, but it is important to some people.