Disclaimer; I don't own Good Omens.
Difference
Crowley reached down a hand to Aziraphale. It wasn't a particularly remarkable hand, as far as hands went; four fingers, slightly scorched from driving the flaming Bentley. A thumb with the marks of someone used to getting his hands dirty. A creased palm that looked like the lines hand been drawn on with a child's crayon, slightly roughened from his brief time in his true form.
Aziraphale spent a split second taking all this in, and then reached out. His own hand was pale, soft, unmarred by scars. It was a book-keeper's hand, with four fingers that specialized in turning pages and smoothing creases. A thumb worn down from endless years of rubbing it across the front cover of his latest book.
The hands reflected the owners; two essentially different people. The difference between them was the difference between Good and Evil, between Right and Wrong, between East and West. One was dedicated to damning souls; the other, to saving them.
Aziraphale knew all this. He knew that if he went with Crowley his death would be even slower and infinitely more painful, allowing he was permitted to die for messing with the Great-But-Not-Ineffable-Plan. He knew that he was damned to an eternity of pain and misery if it turned out this was a part of the ineffable plan.
He took his hand anyway.
After all, the only difference between Good and Evil was what you applied it to. The difference between East and West was the direction you faced. And there never had been any real difference between Wrong and Right.
"Come on. I'll drive us back to London."
They stole a Jeep. Aziraphale realized he didn't care.
If he was going to Hell either way, he might as well do it with a friend.