I purchased those two calculators for the purpose of picking one for my son to use in his upcoming school year.
Well, I got them both and have now gotten around to opening them up, and to my surprise they're the exact same machine under the hood.
Same screen, same buttons with the same functions.
The Casio has a built-in equation solving function, the HP has arguable better aesthetics and buttons.
I guess I shouldn't have been surprised... At least they were cheap!