Dune: Huge map. Fremen move twice as fast as other factions. Atredis and Harkonnen can move triple(?) if they start at their ports. Landing new units outside of major strongholds (like, on top of a spice field) costs double. Placement and movement must be thought out in advance.
Rex: Map shrunk by half. Standard movement doubled. Fremen now only move 50% faster, with move speed less important on top of that. Costs of landing units is flipped so that landing outside strongholds is half as much, making it so you can land armies directly on top of spice blooms to claim them, then move quickly to other spots. Much less advance planning required.
Read over the new Dune rules. I love how alliances are now limited to 2 players. That change was badly needed. 3 man alliances were no brainers, and a 1-2 player team couldn’t compete. Don’t like how The Voice power for the Bene Gessrit player was buffed. That ability was already ridiculous on top of a ridiculous faction (do all their leaders really need to be strength 5?).