Stool softeners come in five different types: suppositories, pills, liquids, powders and chewables. When it comes down to the action they all get the same result just in slightly different ways so it all depends on what you're comfortable with putting into your body (and where).
For really hard stools I would recommend getting a suppository simply because they can get right to the source of the problem very very quickly. I know you're probably reluctant to pop something *up there* but when you're in pain and haven't gone to the toilet for days then you'll soon realize how wonderful one simple suppository can be. The suppositories work by placing a fairly small "pill" coated in wax, that is normally shaped like a bullet, into the rectum. Once inside the wax is designed to melt at body temperature and the active ingredients work to soften the stool and work up through the rest of the bowel. The stool waiting to come out is normally what's causing the problem which is why suppositories are so effective.
For mild to moderate constipation or stool hardness then the other types will be sufficient and it's all just down to your own personal preference as they all have to go through the stomach and through the intestines. These four work by tell the body to not absorb so much water and they also put a thin lining around the intestines to make it harder for the absorption to take place. Having more water surrounding the stool makes it easier for it to become softer.