One of Mass Effect 1's most crucial decisions comes on the planet Virmire, where Shepard is forced to make a choice between saving Kaidan or Ashley during the destruction of Saren's base. This is one of few points in the series in which a Mass Effect companion is guaranteed to die, so the choice warrants some serious consideration on the part of player, and there are several factors to consider when deciding who to sacrifice in Mass Effect Legendary Edition.
While several choices throughout the franchise carry over to later games, such as the fate of the Rachni Queen or the Council in Mass Effect 1, the death of Ashley or Kaidan is one of the more notable moments in the series. The Virmire survivor appears in both ME2 and ME3, and in the third game they can be recruited as a crewmate aboard the Normandy again. Additionally, both of them can be romanced in both ME1 and ME3 - meaning players can expect to see a lot of them regardless of who they choose.
Virmire also determines the fate of several other characters in Mass Effect 1. While he is not a member of Shepard's crew, Captain Kirrahe is a fan-favorite NPC in ME1 and can play a notable role in later games if he survives the events on Virmire. Wrex can also be killed if Shepard is unable to persuade him to back down when he confronts them over their plans to destroy the base. But both of these deaths are entirely avoidable, which set them apart from the crucial Ashley or Kaidan decision.
ME1's Virmire Mission: Who To Save
One factor to consider when picking who to save is the combat aspect of the game. Mass Effect is an action RPG, and combat plays a large role throughout the entire series. If players plan on bringing Ashley or Kaidan along on future ME1 and ME3 missions, they should consider their combat strengths and weaknesses. Ashley is a soldier, meaning she is proficient in combat abilities but lacks any tech or biotic powers. On the other hand, Kaidan has both tech and biotic capabilities. Neither character will be available to join Shepard's squad in Mass Effect 2, however, though they do make an appearance on the planet Horizon midway through the game.
Kaidan is also a romance option for either male or female Shepard in Mass Effect 3, while Ashley is exclusively available for male Shepards throughout the series. Players interested in pursuing Kaidan with a male Shepard later on should consider this when deciding whether or not to sacrifice him. Additionally, while the decision doesn't matter in the context of the game's mechanics, players looking to create authentic roleplaying experience should consider that it makes more sense to save whichever crewmate was sent with Kirrahe's squad - especially if they're aiming to ensure Kirrahe lives to return later on in Mass Effect Legendary Edition. Ultimately, however, the decision is a matter of player preference.