The later shows are always, always less crowded. Parents want to get their cranky kids out of the park before too long, and people have much more time to get to the earlier show (there's only about 40 minutes between when the seating ropes drop and when the show begins for the 10:30 show, whereas people can and do wait for several hours for seating for 9:00).
Here's a tip for judging the quality of your Fantasmic spot. If you look out on the river toward Tom Sawyer Island, you should see six rectangular black boxes floating in the water. Those are the fireworks launch sites, and they're paired. During the show, there will be a large screen of water between each pair of boxes, on which are projected videos that are very important to the show. Try to center yourself between one of the pairs of boxes, especially the center one. Center stage seats are slightly past Pirates of the Caribbean if you're heading toward the Haunted Mansion. You can also use the giant black light towers to position yourself in relation to where the water screens will be.
One of my personal favorite spots is the bridge over Pirates of the Caribbean. If you go about ten feet up the bridge on the New Orleans Square side, you'll have a fabulous, unobstructred view of the stage.
Last tip: be aware if your section is seating or standing. Technically, the center sections are the only ones where seating is allowed. If the show's not very crowded, we don't really mind if you sit in other areas (though you won't see squat, you're supposed to stand for a reason), but if it gets busy and we need the space, we will take it. Ask a cast member if the section you're eyeing is seated or standing.
Cinderella IV: The Bloodening
"It's okay, Beaker, we're scientists. We get paid to fail."