I always go to the nuke site, as I will explain.
From all the various previews that came out about Mass Effect, it seemed very likely that some party member(s) could die on Virmire, so I made sure to save often. This made it possible for me to play out all the scenarios. Strangely I've never messed up saving Wrex, I'm not even sure how you can unless you rush thru the game and or don't bother with Charm/Intimidate.
Anyways my gut reaction has always been to send Kaidan with the Salarians and have Ash arm the nuke. Firstly Kaidan volunteered, and he'd also works better with aliens. Sure Ash might be more useful as a frontline Soldier and Kaidan's tech expertise would be better for arming the bomb, but it plays out better having Ash with the nuke almost always.
When the Geth arrive Ash not really sure what she's doing; panics and arms the nuke. Kaidan not being a frontline soldier would get overwhelmed by all the Geth at the AA tower and dies as a result. This to me makes the most sense in terms of what has to happen.
Now when it's the other way around with Kaidan at the nuke and Ash at the tower... I can just never save Kaidan at the nuke site because it looks so silly to pick up a wounded Kaidan on your back especially if you are playing a male shepard. To be honest it's almost laughable. Saving Ash at the AA tower is acceptable though, it saves more people, Saren confrontation is more dramatic, and Kaidan gets a heroic death scene.
Still ultimately it seems to me most important to make sure the nuke site is secure. Even though there should be no way to disarm the nuke the Geth may be capable of doing so with their technical knowledge. Even if they can't disarm what's to stop them from moving it? It's too risky realistically to leave the nuke unsecured. Plus why would Saren go to the AA tower just to confront Shepard? If Saren knew the AA tower was a distraction, you would think he would go to the nuke site, kill whoever was there personally.
Not only that, but I prefer to go to the Nuke site because the fight there is a little more challenging what with the Geth Prime and all. The AA tower is kind of too easy.
Lastly I usually save Ash because it seems the right thing to do. Kaidan doesn't really have a family or much of anything to live for except some long lost love. Kaidan also seems much more at peace when you let him know he's probably not going to make it. Ash on the other hand has a family that kind of depends on her, and she has yet to clear her family's name. Plus letting Ash martyr herself as a result just doesn't seem quite right. Kaidan is also older so in a sense Ashley in comparison has yet to have lived a life as much as Kaidan.
Ultimately I like both characters; Kaidan for his cool and calm outlook on things as an L2 Biotic and Ashley for her spunk as a female Marine. Personally I prefer Kaidan as a team-mate since I like to play as the default Shepard which is a Soldier, so Kaidan's Tech/Biotic abilities are really handy.
Yet I always save Ashley cause it seems wrong to let her character die.
(Regardless of whether you are romancing her or not as well. I prefer Liara's romance and I like to get my Captain Kirk on with my Shepard.

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Still it's one of hardest choices I've had to make in a RPG, and for that I have to give props to Bioware. The fact that you actually care something for the characters shows just how well a crafted game it is. Guess that's why I always like Bioware games so much.