I am really impatient and hate to stand in one place for a long time. On our first visit to WDW/Epcot, I could not bear to wait around so that I maybe could catch part of a glimpse of Illuminations through 20000 heads. The second trip I made a reservation for a private cruise so that I could finally see the show. My fiance and I LOVED it. For us, who love to really explore the parks and see all the small details, it was so worth it to hop on the boat and have an up close view of the show. We went our last night, which was good for two reasons.
1) we weren't nervous about trying to see the show because we knew we'd have to watch it. No squeezing it in anywhere
2) the build up was great. We spent many evenings at Epcot, but we promised not to take any sneak peeks so that we could be totally suprised on the last night
I can't help with the fireworks cruise, we've never seen the fireworks at WDW (we have at DL a number of times). My gut feelings is that b/c they're up in the air, it's easier to see from anywhere, where Illuminations involves stuff close to the water...it's key to have a good seat
hope this helps