One is a blue hedgehog with massive kicks, and the other is an ancient dude in a sea of ancient dudes.

Of course the distinct one is more recognisable than the regular human-looking one...

I've seen many, many pictures of Shakespeare and still probably couldn't recognize him. A blue hedgehog? Well, it's literally a blue hedgehog.

Well duh. One's the main character of a still popular media franchise, the other is a long dead poet whom most people have never seen a picture of and only know because they studied his plays at school.

Of course they're gonna recognise Sonic over Shakespeare, that's true for most people living in the 21st century!!

Like Sable from Animal Crossing? (Every time I see her, I'm surprised Sega didn't sue over her. She and her sisters are all hedgehogs. Only Sable is blue. I would never mistake her for Sonic, because she's a girl, and wears an apron, but still, anthropomorphic blue hedgehog.)

On a similar theme

Sonic wrote plays at school?

Sega would know they don't have a case. "Blue hedgehog" is too vague to be trademarked.