The main difference is that of detail IMO, both in styling and in shading.

Try looking at all the little things the hair in the render I posted is doing. It's got little clumps in it, it's got areas where the hair is tufting in different directions, it thins out at the hair line in a believable manner, it flows smoothly into the beard (which is a totally different style of hair BTW) and it looks soft and fluffy like real hair.

Now look at Poser hair... it's basically all variations of bob cuts or afros with very little detail in hair structure and style. Where are all the subtle little things hair does in real life that make it look like living human hair ? Also look at how poser renders hair as if it were stiff straw or thin metallic wire (even with ridiculously long render times)