To me, stems are commodities. The decisions factors are simply:

Essential:
clamp size
length
angle

Totally personal preference:
Price
Weight
looks

Get one you like the looks of and can afford in the size you need. The weight difference between a relatatively cheap one and an expensive one will be 160-180 grams compared to 100 - 120 grams, so it's nothing at all to worry about unless you're trying to achieve a certain weight goal.

I have two - chosen only because they are pretty high end aluminum stems, but I found them pretty cheap on ebay: a Ritchey WCS and a Syntace 99. But they function no better or worse than stems I could have gotten for 1/2 the price.