This subject becomes more of an emotional one (than an objective one) for me.

The novel goes into some minor detail about how Tessio was smarter (than Clemenza). He picked up on things more quickly (example being when the Family was organized Vito wanted Tessio and Clemenza to be distant from each other so they wouldn't be in a position to jointly overthrow the Don. Tessio understood this, and was given "more room" to run his regime freely, while Clemenza was held on a tighter rein).

As I see it, Tessio's "mistake" was that he didn't trust Vito's judgment enough to follow Mike. Clemenza did, and we know the rest.

Nobody likes a traitor, and conversely everyone likes a loyal friend. Thats why we "look down" on Tessio, and appreciate Clemenza (this is why its an emotional issue for me).

"Can't do it, Sally". Four sweet sounding words.