If you think Knuckles could be more of a Sonic clone than Shadow would be, you are mistaken. Shadow in his first and just about all appearances when he's not primarily using stupid slower-than-running vehicles and guns with terrible targetting is basically just a carbon copy of Sonic. Chaos spear is not enough of a difference for two characters that are essentially identical in their gameplay. If anything, he's typically the same as Sonic without all the goodies that Sonic gets (Like the bounce bracelet in SA2).
Knuckles on the other hand plays differently enough that it doesn't feel like you're ever playing as Red Sonic. He glides, he climbs walls (probably wouldn't translate well to Smash if he were in fact used), he digs into the ground, busts through walls by punching them, he runs slower, and jumps shorter. The only thing that he has in common with Sonic in the classic series is that he can spin dash and he can damage enemies by jumping/rolling into them.
tl:dr
Shadow is more similar to Sonic than Knuckles
Shadow is usually a carbon-copy of Sonic in most games (minus gunplay/vehicles)
Knuckles functions differently both in terms of abilities (Gliding/Climbing/Strength)