I've done Dining Package, FP+ (most recently 8 days ago), and Standby. From what I've seen, it seems like they block off half the theater for Dining Package & FP+. Dining package is in the middle, and FP+ goes from the end of dining package to the end of the theater. Dining package were the best seats we had, but we've also had decent seats with FP+ when we arrive earlier. And we've never had trouble getting F! as a 4th FP+, even as late as an hour or two before the show (last week we saw the 9pm show, which was the first of two shows, and picked up the FP+ after getting on Alien Swirling Saucers at 6:something).
That said ... even with FP+, you have to arrive early for the better seats. With the dining package you're guaranteed to be closer to the middle no matter when you show up (you just might be all the way in the back).
I'd say if you're interested in a pasta lunch anyway, go for the dining package, but if you're interested in lunch elsewhere, you can get good seats with FP+ if you show up as soon as the window opens.
Just maybe don't show up for your FP+ 10 minutes before the show, because then you'll be all the way at the end ... although that's definitely just a guess and not something I did after a way longer than expected Rock n Roller Coaster line. (All the way at the end wasn't a terrible view, it just wasn't a great view)
It's funny, I seem to be the only one who feels this way, but I prefer F! at WDW. When I saw it at DL it made no impression on me and when I saw it at WDW a few years later I didn't remember ever having seen it before (in fact, I was somewhat certain that I hadn't seen it at DL ... until I realized I had pictures).
Maybe it's because I saw F! at WDW after I saw World of Color (my favorite night show ever), and some of the water effects and projections reminded me a bit of WoC.
I know I'm in the minority here, but I do feel that it's worth seeing at WDW, even if you've seen it at DL.