There's always going to be similarities among "end of the world, good vs evil" type stories.  Whether it's the Dark Side of the Force, the Black Ajah, Deatheaters or a myriad of other "evil minions" from other storylines .... those characters are pivotal in creating the part of the story where it gets so bad you don't think it can get any better and then, somehow, Good Wins, Evil loses and we all walk away happy.

Since stories are usually centered around the hero(es), you don't usually see a lot of the bad guy until the very end .... but it's really no story with just reading about the hero so you need to create these minions to not only build drama for the reader but also develop the character of the heroes themselves.

As far as HP vs WoT .... 2 totally different types of stories.

I look at like this - there are, in my mind, 2 different type of authors (and thus stories):  Shakespeare and Tolkien.

Tolkien built an entire world.  His own languages, back stories, world history and brought his characters through that world.  Using the depth of the world to enrich the "reality" of his characters.  You could that, in this world of his, someone like Frodo Baggins really existed and you could saw how he (and the world) got to the place it was before we even began reading about them.  From there, the story brought us into that world and we could feel more like we were a part of the Fellowship with a fuller understanding of the land and world around us.

Shakespeare did not bother with vast back stories.  He devised excellent characters and provided great stories that unfold before the reader as they go.  You don't need to know all the geography or history of Hamlet's country.  We don't need to know the technical details of how his father's ghost comes into being.  We don't need to know all of Hamlet's back story.  We just need to know the bare minimum of what conspired to put him in the predicament he's in and what happens from there. 

Both authors were great in their own right but both took very different approaches to writing their books.

Jordan and Rowling are similar in nature.  In Jordan's world we learn much about the One Power and how the channelers use it and limitations it has.  In Rowling's world, magic is just that .... magic.  We don't get to know how it works or how new spells are developed.  It's a mystery that she doesn't bother to delve into because the focus of her books is solely on the characters and the story they portray.  4 high school students can make a map that shows everyone in Hogwarts but the Ministry of Magic (with all their aged wisdom) can't create anything similar to spot when someone like Voldermort shows up in their area?  Seems like a big hole in the story and there's several others when you stop to look for them .... but HP is still a great story that keeps you intrigued just as much as WoT.

Different authors..... different techniques .... 2 great literary works .... but no way to compare them side by side and say "this one is superior"