I suppose hoped reduction of pain comes first but I think all the variables interplay really. I suspect hanging would be more painful and distressing than SN. But then, I'd consider hanging more if I knew it was 100% reliable. But really- none of the DIY methods feel 100% reliable.
I think the other major issue is accessibility. Who wouldn't choose nembutal if it was easier to access and, legal? There are maybe more peaceful, reliable and quick methods for many of us but, we might not want to risk breaking the law to get them.
I'm tempted by inert gas in terms of reducing pain but certain things about it trouble me. Obtaining and storing the equipment. Not hurting others. I think pain is a factor for a lot of people but, I think lots more comes in to play. And, we'll compromise here and there to accomodate.