will vary depending on the technical style - some lifters are more knee dominant and therefore more vertically oriented; others are more hip dominant and therefore more extended. i like to see legs approximately vertical and shoulders slightly behind the hips, i.e. hips are a bit hyperextended, but the hips are not pushed through the vertical plane extending up from the feet. clean extension will generally have slightly less hip extension than snatch.
I think either extreme is less effective than some combination of the two ends, but I wouldn't say one is better than the other - world records get set in quite a few different ways.
As far as genetics vs acquisition, I'd say it's likely both. Athletes are suited for certain styles based on proportions, etc and will naturally gravitate toward those styles, and then after training that way for a while, it becomes even more ingrained.