In their early years I found Zoot to swing a bit more than Al, but that Al was slightly more creative in his solos. The difference though between them was slight.
Over time I found the differences between them became more apparent partially because the sound of each of them deepened and took on a less Prez type sound and had a more unique character. Al's sound became more in the vein of a Jewish cantor. It had a wailing quality that made it identifiable for me. Zoot's sound also became a bit deeper.
Though some here will disagree, to my ears Zoot's solos became more interesting in his Pablo years. He still swung hard, but with a bit more creative depth.
It was easier for me to identify the difference between them in their later years than in their earlier period.