One's a "city", the other is a military base of operation. Toy wise, originals are quite different even in size, the Generations and Titans Return are very different, but (thanks to those that posted pictures) I can't even imagine confusing them. In fiction, I remember FM being used as a regular character for a while in the Marvel series, but Metro was barely touched. I never had any confusion over them, not even sure how one can as they really are very different, even in black and while, heck their silhouette are absolutely different so you should even identify them by their shadow. Maybe you never looked at them side by side?