As an artist I can answer that...
here's the difference...
McCartney produced a huge body of work, but in listening to all of it... I have no idea what the man is like.
He has left us no clear impression of himself, of what moves him, of what he believes.
Lennon, on the other hand, left an indelible impression of his true character, both the positive and negative aspects...
And beyond that, his body of work describes to us the arc of how he was changed by the world around him...
We come away from Lennon's work with an intimate understanding of the man.
That is why Lennon's death prompted such a vigil, whereas Harrison's did not
McCartney can bang out a great tune... but his most compelling work was done as a Beatle, with Lennon writing the edgy, emotional bridge.
Lennon would take a sappy love song and give it a twist, exposing the pain, the longing, or the despair inherent in love.
And it is clear in McCartney's post Beatles music how much influence Lennon had on McCartney's creative output, and vice versa.
McCartney's subsequent work lost most of its edge and nuance.
While Lennon's lost much of its musical polish and pop appeal, but Lennon never lost the deep emotional honesty.
I think there is no doubt they did their best work as a team, each driving the other in the areas in which they were weak and supporting each other with their strengths.
( McCartney and Lennon both have commented that, even in writing songs without the other's input, they were each striving to impress the other)
And I am not saying McCartney is not a talented musician working in the arts...
But the difference between Lennon's work and his is like the difference between Hemingway and John Grisham, like the difference between Picasso and Normal Rockwell.
When it comes to art, Rockwell was well loved, but Picasso defined what art was to become.
Grisham is poplar and widely read... but Hemingway defined the future of what great writing would be.
They both created musical art... but Lennon's was far more emotionally impactful... and that is what makes art great.
And Lennon's art extended outward from music, into poetry, cinema, protest and social activism.
I remember the billboard, all across america... War is Over if you want it
Where the fuck has McCartney been in moving a generation?
God how I wish Lennon had lived.... Iraq would never have been invaded with him to call on our consciences...