1500 is more commonly preferred over 2000, kimmikayb.
It's closer to the core Pool/Trading Post/Campfire area. It's a smaller loop (20-some sites versus almost 70 in 2000). 2000 is also famous for weekenders that come in and "take the place over" and group campers that didn't get in the adjacent Creekside Meadow group campsite.
1500 does get some small groups too but not to the degree 2000 does.
1500 sites are hard to get because they are in such demand. Try to arrive early in the morning, offer to wait for a site to become available, or check in on a non-Friday or non-Saturday. Those are all steps that can improve your chances of getting 1500.
But remember it's only a request and not a guarantee.
Bama Ed