I don't actually 'think' about my choice at all. I look through the pictures, spending about 5 seconds on each one, and one or two will just stick out for no quantifiable reason. It could be the brand, the colour scheme, the condition, the componentry choice, or just the nostalgia factor, or maybe a combination of several things. If the choice still isn't obvious to me after a second perusal of the pics, I won't vote.