Citron does have a more "pure lemon" fragrance than lemon, in my opinion.
What I mean is that if there was a spectrum between lemon and mandarin, citron would be even further away, slightly past lemon, from mandarin.
The fragrance of citron is nearly the same as lemon, but just a little more "pure". When you smell citron, you realize that lemon has maybe a hint of mandarin/orange smell in it after all.
I would also say that citrons may be somewhat slightly more fragrant than lemon as well, though I could be wrong about this.
(It is a cleaner, more ethereal and less substantial smell in quality though, so actual intensity may not be quite the same thing as perception)
Those who are less discerning and do not pay close attention might say the two smell exactly the same.