Difficulty level is relative
I want to take a few minutes to talk about "Difficulty Level". One of the first things I noticed when I started playing Sudoku is that almost every puzzle is printed with some sort of rating.
Sometimes you see 1-5 stars, words like "Easy", "Medium", "Evil", etc.
The problem is, I have found that these ratings usually don't mean much. I have done puzzles marked "Hard" that in my opinion were easier than a "Medium", and vice-versa.
Unfortunately, what one person would consider "Hard," another might see as "Easy." This is because each person has their own techniques they are good at.
Even worse are puzzles in most newspapers. Once you master a few of the basic techniques, most of those puzzles seem "Easy" no matter what the rating.
One of the best ways to rate a puzzle, in my opinion, is not by counting how many cells are empty, but by analyzing what techniques are required to solve the puzzle.
Of course, you don't know what techniques a puzzle will require until you solve it, which is a problem. Fortunately, there are computer programs out there that will do this for you.
I have tried most of them, and the one I like best is Sudoklue. It has an "analyze" feature that not only gives a puzzle a general rating, but more importantly tells you what techniques you will need to use to solve it.
If you want to know how hard a puzzle is going to be before you begin, I recommend you ignore the rating on the puzzle, and use a computer program to analyze it for you instead.