From my own experience, I think it's harder to say you don't.
That you don't
Depends. If you love them, it can feel totally natural and easy to say it.
If you don't, but you know it'll hurt the person if you say it, it's hard.
It's also hard to say you love someone if you know/fear they don't love you back, in some desperate attempt.
Depends on who ur saying it to and how much u care about their feelings and how much u know they feel about you..
I think we sometimes over use the phrase, "I love you."
I think it is harder to tell someone you don't love them especially if they wish you did.
Harder to admit when you don't.
Depends on what you are more afraid of, rejection or guilt.