I am probably repeating some things that have been said before, but I wanted to express my own point of view on who was of a "higher" rank.
To me, comparing the WK to MoS is like comparing Apples and Steak. What, you thought I was going to say Apples and Oranges, eh?
*ahem* Anyway, they seem to have two rather different roles in the service of Mordor. MoS is a diplomat. I'm sure that he could be a rather big pain for anyone who wanted to fight him, but his main set of skills seemed to be those of negotiating with (or coaxing) others. Perhaps his "great sorcery" could be much of the same as Saruman's... a voice that can enchant you and sway you even if the arguments it proposes are flawed, or not in your best interest. And why not? We have (as far as I recall) examples of three sorcerers in Lord of the Rings.
Gandalf did some (minor IMO) conjuring, creating a big fire when the wolves attack the Fellowship after their failed attempt of crossing Cahadras, and also trying to hold the door closed against the orcs (and then the Balrog) in Moria. But when it comes time to fighting, what he mostly does is wield his sword... and
give good advice.
Saruman, instead, only used his voice. Even if his power was partly spent, I am sure (and Gandalf hints at it in the books, sadly I'm not using my computer so I can't look up the quote) that he
could have done a few "nastier" things, but he chose to try and sweet-talk them. Perhaps because that was his most "powerful" magic? After all, in Tolkien's world it seems that you can only throw so many fire-balls, or hold a door shut for only so long. It takes a toll on you, it's not the computer game-style wizzard that throws fireballs left and right.
So perhaps MoS' great powers were not in humongus fireballs, turning his foes into stone or wielding a sword of fire and ice. Perhaps his great powers were those of corrupting the unsuspecting mind, weakening the will of others and turning the hearts of otherwise noble people. That kind of 'magic' is the one (I think) we see the most in LoTR. Even when Aragorn, Legolas and Gimli are chasing after the orcs that have Merry and Pippin captive, they say that there is an "evil will slowing them down and speeding their enemies". Saruman's Magic? That's what they seem to think it is, as they say this "will" comes from Isengard.
On the other hand, the WK had a very different set of skills. He could still command and dominate other people's wills but in a very different way. WK, as all the other Nazghul, but him even more so, was scary.
Really scary. He could scare you into submission, but since the fear he caused did not seem to be something he did on purpose, but rather an innate skill of the Nazghul, I don't think he'd be much of a diplomat. Sure, he'd get short-term results (ie. Whoever talks to him will probably be scared into following his will) but he will not affect a change in this indvidual's mind as MoS perhaps could. Whoever was coaxed by the WK would think "I better do it, else I'm dead" rather than "I want to do this, I want to serve my Master Sauron".
But then, WK's skills were better suited to the battle field. It doesnt really matter if an orc runs himself against a wall of shields held by many strong and able fighters because he "wants" to or because he knows that whatever happens to him, not following the order will have even worse consequences. The only thing that matters is that it gets done.
That's IMO why the WK and MoS are so different that can't be compared all that much. We have to work hard to remember (in spite of the best efforts of gaming companies against it) that Tolkien's world was very much fluid, there were no set "levels" of skill that the characters had, and no-one was so far above the rest that he was virtually invincible. After all, Saruman (whom Gandalf still considered a possible great ally, even after his fall) was killed by the lowly Wormtounge, Sauron, a demi-god was defeated by Elendil and Gil-Galad, even Melkor was hurt rather badly by (was it Fingolfin?).
The question "who was stronger?" in Tolkien's work, to me, is a lot of guesswork as there are a bunch of variables that must be taken into account. It would be like asking "who is more powerful, Farael or Dancing Spawn of Ungoliant?" Well, are they armed? if so, are they good at using their weapons? if not, who is physically stronger? who is in better shape? is either of the two likely to hold back for some reason? will they be alone or with an army of minions that will do the fighting? etc etc.