Both Metro 2033 (which also makes its PlayStation and Linux debut) and Metro: Last Light are now enhanced to run at solid 60 frames per second on consoles along with featuring full 1080p/912p resolution on PS4/Xbox One respectively while the PC version supports even further enhancements such as support for 4k resolution.
Both games now feature Global Illumination for more impressive lighting and terrain tessellation among other upgrades of the 4A Engine. The PC release dropped support for 32-bit operating systems to improve overall performance even on lower-end machines. This is at least part of the reason why Redux could not be offered to owners of the originals in as an update.
Two new difficulty modes have been added to both games as well, Spartan and Survival. Spartan is based on the Last Light gameplay experience, while Survival is based on Metro 2033 and adds to the challenge with extremely scarce resources, more aggressive enemies, slower weapon switch and reload time.
Both games natively include the Ranger modifier, which was only available as separate DLC in the original releases of Metro 2033 and Metro: Last Light. Ranger Mode again strips away the HUD completely and makes the player rely solely on the in-game indicators - such as Artyom's watch and each weapon's magazine. Damage output to both player and enemies has been increased, meaning one or two-hit deaths/kills ought to be taken into account when assessing combat encounters. Ranger Mode can be added as a modifier to any difficulty mode.
The Achievements and Trophies from the original versions do not carry over into the Redux versions. In fact, some achievements were cut and new ones were added. Metro 2033 Redux and Metro: Last Light Redux will have separate Achievement/Trophy lists on their respective systems with 50 achievements per game. On steam, the Redux versions will appear as their own, separate entries when the player already owns the original releases meaning the originals can be played in their untouched form at the user's whim.
Both Metro 2033 Redux and Metro: Last Light Redux are available separately as digital downloads on PS4, Xbox One and PC but they are also sold together in a physical release, simply titled Metro Redux, on all three platforms in Europe (in North America only the PS4 and Xbox One versions are available at retail as disc versions).
PC players can benefit from an upgrade discount if they own either one of the original games or both on steam: