Thanks to the magic of video game licensing, some games have received more than one official port for a system. This is usually seen on computers, where there is often more than one way to load a game into the computer's memory. If one company licensed a game for release on cartridge, another company could still license the same game for release on disk. Zaxxon is one such game, with two releases for the Commodore 64: a cartridge by Sega, and a disk by Synapse.
Sega's cartridge release isn't bad, but it isn't really great either. The graphics look decent enough, but actually playing the game can be rather frustrating. The biggest problem is inconsistent speed of the player's ship and laser shots. Sometimes the ship moves with a snap, diving almost instantaneously, while other times it seems bogged down in molasses, barely able to lift a wing. This problem is made worse by the fact the ship's forward speed never changes, making it difficult to judge when there is enough time to navigate around obstacles like walls and force fields. The game has been ported fairly faithfully otherwise, even if there are a lot of little differences. The side-facing laser cannons fire even in level 1, but the parked enemy ships never fire. Officially the enemy fleet has only 15 ships, but even when you're down to the last one or two, you still see a lot more of them in space. The game would still be good enough even with these other differences, but the speed problem definitely hurts.
Better is the Zaxxon disk published by Synapse. This version is guilty of many of the same changes as the Sega cartridge, including the sideways cannons firing from the very start and the parked enemy ships never firing at all, but game play is a whole lot better, thanks to consistent speed of both the player's ship and laser shots. The graphics are noticeably different from Sega's, but not really better or worse. There are still some annoyances, such as the occasional homing missile that appears even when the player's ship is kept close to the ground, and some very odd movements of the enemy ships out in space. There is also the interesting feature of the asteroids getting more populated with enemy weapons in the higher levels of the game, giving the player more cannons to dodge and more radars to blow up. Even with these changes and oddities, Synapse's Zaxxon remains a decent and satisfying port, much more so than Sega's offering.