We'll be in MK this Wednesday and our touring plan finishes up at 5:30pm. Which evening show should we see? We'll be at AK on Thursday for half the day and then HS in the afternoon and evening. Fantasmic is my all time favorite Disney evening show, so even though we don't have a FP for it, we'll wait on Thursday night for that. For Wednesday, we've seen Happily Ever After a few times and thought it was meh. Other favorites are Illuminations and the Star Wars Spectacular. We've never seen Rivers of Light. Would you recommend going to AK after MK on Wednesday to wait in line for ROL? Or, is it too risky that we wouldn't get in? The other option would be to go over to Epcot for Illuminations. The ROL and Fantasmic dining packages aren't in our budget.
Since you've never seen ROL, I'd go for that if showtime is late enough that you'll be able to get in standby line at reasonable time. If your FP at MK are early enough, you might even get lucky and be able to pick up ROL FP.
I would see ROL if you have not seen it, but we were lucky enough to get a fastpass so I can't report on wait times for standby. We saw people standing in line for standby early, but there are a LOT of seats, not sure how necessary it is. And honestly, you can likely see from outside the stadium seats - just not my favorite show. But I'm not a huge Illuminations fan, either!
I'll preface this by saying that I have not been since the show opened. One side of the seating is reserved for FPPs, the other side is for dining packages and has some standby seating. I've read that the number of SB seats is a function of how many dining package seats were not sold.. Exact numbers aside, as large as the seating area is, only about 25% of it is typically SB. I was monitoring this pretty closely when I though I had a trip a couple of months after it opened, but when the trip fell through I stopped tracking it very closely. Back then, the word was you had to line up at least 90 min prior to show time to get a SB seat. It's been playing long enough now that I would assume that number has gone down a bit; perhaps someone with recent experience can comment. I've read that there is a pretty good view of the show from the bridge between Dinoland and Asia, but I am guessing that those spots fill in pretty quickly.
ROL aside, AK is a beautiful park at night. A night ride on EE is awesome, you might be able to catch a night KS, there is live entertainment at various locations, and Pandora is a whole different experience after dark. For those reasons alone, were it me, I would go to AK after your MK day. There are also some good QS options at AK for dinner.
EP, especially WS, is one of my favorite things to do in the evening. The films in China, France, and Canada, The American Adventure, and the boat ride in Mexico are all fairly easy to do without advanced planning (FEA is a different story). WS has some of the best dining options (TS and QS) in WDW, and it's pretty easy to find a decent spot to watch IllumiNations with only 15-20 min of waiting. Prior to the PM offerings at AK, EP was always my "go to" location for an afternoon hop.
A third option, which you didn't mention, would be to go to DHS to catch the new Star Wars PM show. You would be there in plenty of time to find a good spot for that (although I don't know how much lead time you would actually need). For me, the downside of this would be dinner; I find most of the QS options at DHS to be pretty dreadful, and the chances of getting a last-minute ADR or walk-up at any of the TSs would be pretty slim.