You know... I loved shadow. I know it got as much hate as the next sonic game, but dang was it interesting, multiple storylines, super attacks, guns, and it still had the speed thing. Never played 06 but heard it would've been alright if not for the outrageous glitchiness
Doesn't really matter which is better since they're both vile shitstains on the franchise's reputation.
At least Shadow was just a spinoff, 06 pretty much left a permanent mark on the franchise so damaging that, to this day, it still hasn't been able to shake off the after-effects despite Sega's (half-assed) attempts. It's the reason why Sonic has been the only playable character in a 3D title for the last 13 years and the reason why Sonic is meme'd to the teeth and treated like a joke.
...I guess swinging around a scythe with Shadow was fun... Finding limited entertainment in having him use weapons that he doesn't actually need is all the replay value the game really has.