The Nazgul and the Dead Men were quite different in nature. I'm not sure how similar you could say they really were, other than the fear they struck in their enemy.
All the Nazgul had a physical form, that was bound to Sauron, and for his use. How do they sport nifty black cloaks without a body? It was invisible, but there was definitely a body there. Pierce it, just as Merry and Eowyn did to the Wiki, and it would die. Due to their bond to Sauron and the One Ring, they could only be killed under unique circumstance, but a physical presense nonetheless. Their bodies had just faded because of the 9 Rings, as Gandalf said would have happened to Gollum had he continued to keep the Ring...his body would have faded, but he would still have a physical form.
The Dead had no physical form (or if they did there has not been evidence for it). Gimli tells his companions he doubts their swords would have any "bite" in them, and he was most likely correct. Their bodies are gone, only their spirits remain, and their spirits were barely visible simply because of the power of the oath. If you look at Isildur's curse, he tells the Oathbreakers that their
spirit would never rest until their oath was fullfilled. So, even many many years after they have died, their spirits are bound to remain because of their broken oath. A spirit without a phsical form, can not directly cause phsyical harm...case in point Sauron, who after his defeat would be a wandering spirit never able to reform again.
Now could Aragorn have continued to use them as a weapon of fear? Maybe...and how successful they would have been is anyone's guess. The Wiki has controlled dead spirits before, so I doubt he or any of the Nazgul would be freaked out by 'em. And eventhough the Wiki has shown control over dead spirits, the Dead Men were under Aragorn's power, because of an oath. Oaths in Middle-earth are very powerful things, and I can't imagine the Wiki having the power to override an oath.
