Armin’s Colossal Titan looks sadder, and has lips, rather than Bertolt’s meaner look.

(Edit:)

There’s also no nose on Armin’s Titan, and his neck is longer and more exposed than Bertolt’s. As well as looking as if he has less teethe.

(Edit x2:)

Armin has no ears as well!

(Edit x3:)

Long overdue, but…

!!!SPOILER WARNING!!! … lol