Since you've got them in ascending order, take every other prime factor and multiply. Compare that to the product of the ones you didn't choose. Whichever's closer, there ya go. That's just off the top of my head.

Let's try another way. Pretty clearly, you want roughly half the factors. You can work from the outside in, or the inside out, or do something like this: If you have N factors, take the (N*2)th root of the number. Find the factor closest to that. Divide your number by the square of the chosen factor, decrement N, and repeat until you get further from the square root instead of closer to it.