For a Mazda MX-5 1.8 (2003), what's the difference between DeatschWerks pumps 9-101-1000 and 9-101-0848?

The only difference is the inlet filter/sock fitted to the pump; all other characteristics are identical. In general you can use any pump with the same body shape. Alternatives are the AEM high-flow in-tank pumps (e.g. 50-1000, E85 50-1200).