They are both powerful wizards, but both differs in some ways. Hermione is a brilliant witch, and as Hagrid says in book 1:
“An’ they haven’t invented a spell our Hermione can’ do”
...which is true. She's so young and has already learnt all spells and books by heart. Plus given the fact that she's a Muggle-born, which means everything she did on Hogwarts on their first day are her firsts too.
Meanwhile, Harry, who is also a not a Muggle-born, but was raised by Muggles and does not learn all the spells and books by heart. He's no better than Hermione when it comes to exams, but he defeated the Dark Lord. Which makes you wonder how come Harry is a more powerful wizard than Hermione?
You need to understand that Harry is meant to deal with the Dark Lord, and Hermione isn't. I bet that if Harry died, Hermione would still be able to survive, and might even defeat the Dark Lord, because she is a one excellent witch! But hey, let us never forget that Harry won't be able to survive all throughout until the end without the aid of Hermione and Ron, we all know that.
Remember when they went to the forest to hide? Hermione's the one who performed the charm to protect them.
She remembered that the Devil's Snare is afraid of light on Philosopher's Stone.
She solved the logic of the potions made by Snape on Book 1.
She created a PERFECT Polyjuice Potion, AND IT'S HER FIRST TIME DOING IT.
We could go on a very long list on how many times Hermione saved that powerful wizard who defeated the Dark Lord. She's not the one who defeated Voldemort, but the long list will prove how great and excellent she is.
And by the way, I have no idea on the measurement of who's greater than who. It's like comparing Dumbledore and Voldemort. Voldemort is a great wizard, but did bad things. While Dumbledore is, too, but did not go on the Dark side. But they say (and I believe because it's what I want to believe) that Dumbledore is much greater that Voldemort, for he is the only one Voldemort is afraid of.
It's who you want to believe that is more powerful. But that would only be our opinions or views.