Good question! They're basically the same setting with two different rule sets, and QZ has a lot more material because it's a longer game. My goal is that you could use the content from either with the rules from either. To summarize what you'd find in each....
QZ
- 70+ pages (revised edition on the way with 90+ pages)
- Into the Odd rules – hit points, 3 ability scores, saves, attacks with no to-hit rolls
- 100 alterations
- 20 compulsions to make you go adventuring
- 60 contacts split between military, science, and underground
- 100 encounters split between 5 regions of the zone, each with generators for features
- 20 sample artifacts plus random artifact generators
2400 Zone
- 3 pages + cover (deliberately super short and quick to read)
- 24XX rules – polyhedral dice with partial success, with no hit points, ability scores, or separate combat rules
- 36 alterations (redundant with the list of 100 in QZ)
- 20 contacts split between military, science, and underground (different from the ones in QZ)
- 20 encounters (some with vague overlap with some in QZ)
- Artifact generators (no sample artifacts)
Each game also has some generators for anomalies, but QZ's approach is likely to see some significant updates when I finish the revised edition. (I'm hoping to be done with that this year, but the pandemic, parenting, and my day job make that tough to predict.)
Hope this helps!