Prepare for a short essay about Virmire Incident...
Since I don't really give a toss about either Ashley or Kaidan, I approach the Virmire situation from a military perspective to make my decision.
1. To decide who to send with the STG to act as a liaison between Captain Kirrahe and Commander Shepard and possibly offer help in leading the Salarian Task Group team, the options are:
-an experienced Staff Lieutenant with training in leadership (he's a commissioned officer), tactics, biotics and technology
-a Gunnery Chief (Non-Commissioned Officer) recruited to the team from an otherwise destroyed unit originally stationed on Eden Prime
The Salarians don't need another soldier, they need someone to manage the co-operation between the STG diversion group and Shepard. Kaidan gets the job. Ashley doesn't have much to offer in this job compared to Kaidan.
Therefore, I will assume that Kaidan is the "right" choice to co-lead the STG team, and Ashley's position will be with the nuke.
2. This automatically means that Ashley will be delegated to the job of arming the McGyvered nuclear device.
3. After the nuke has been armed, STG and Kaidan call in for assistance. Shepard and his team responds, departing the bomb site to head for the AA-tower to aid the Salarians and Staff Lieutenant Alenko. The decision point appears as they are already a good way from the bomb site, after Shepard had already made a decision to go assist Kaidan and the remaining STG.
Now, technically at this point Shepard has arguably already made a mistake - leaving the bomb site with insufficient force projection to secure it. Unfortunately this puts the player into a position where they have to make the most out of a bad situation. The options are not actually to make a choice between Kaidan and Ashley; the options are as follows:
-ensure the successful detonation of the jury-rigged nuclear device, or
-risk the bomb site protection in Ashley's (dubiously) capable hands in favour of saving the STG team and Kaidan at the AA-tower.
The former is more mission-critical, while the latter can result in more lives saved.
Personally, I consider Shepard's split-second decision to head for AA-tower a tactical mistake, but after s/he has done it, one must consider the fact that second-guessing or indecisiveness on the battlefield is one thing that'll get you or your team killed more likely than anything else you could do (or not do). Typically, doing anything is better than doing nothing. Staying on the move is preferable to staying put, and after a decision has been made, it's usually best to follow it through rather than change your mind en route to return to earlier position.
Hence, I usually consider that Shepard made his or her decision when s/he left the bomb site for Ashley's protection, and therefore logically s/he should follow through with that decision, go to AA tower, and be done with the mission.
Also, because of shameless exploiting of game mechanics, the bomb will explode either way, so I usually go for the AA tower in order to save more lives (there is also a role playing element here; my character is typically a Paragon, so that would likely be the way my Shepard would think).
From this standpoint, it's clear that Ashley is the character that should get nuked as the "default" option.
This deductive process may or may not be influenced by the fact that I don't like Ashley's character even as much as I like Kaidan; at least he has Carth's voice to amuse me, while Ashley is like a living Bible Belt standing in the Cargo Bay (plus I don't have any use for her on any missions after Eden Prime, as typically my Shepard has enough combat prowess that I don't need a full soldier in my team - either my Shepard is a soldier and team is Shepard/Liara/Tali, or my Shepard is an infiltrator and team becomes Shepard/Garrus/Liara, which usually works pretty well.
It is a mystery to me why Shepard didn't secure the bomb site him/herself with the present team, and send the remaining ground team to the AA-tower. But that's one of the game mechanics that just have to be accepted, I guess - like the fact that Ashley won't fail to protect the bomb in any case so any risk about the mission being compromised is convenient nonexistent to gear the "decision" toward a character preference based choice.
