I'd have to respectfully disagree.
Pretty much every time I've ever seen a Trek good guy actor try to play a Trek bad guy or heavy, it's often been shite. Overly hammy, or otherwise being the Diet Coke of Evil....just one calorie, not evil enough. About the only exceptions I saw was Brent Spiner as Lore, or Leonard Nimoy as Mirror Spock.
Tom Hardy, in my humble opinion, rocked as Shinzon. Patrick Stewart is a fine actor, but I don't think he could've pulled off the subtlety that Hardy did.
I see why we disagree. I didn't see Hardy's performance as anything beyond average. Most would say that's because the story/script was lackluster. I submit that Stewart has superior acting chops and knowledge of the Picard character to not only pull off the "evil twin" role, but to make it truly memorable.
My main desire for seeing Stewart in that role instead, however, is because I didn't
believe Hardy's version was Picard's clone. Not for a second. The big reveal of Shinzon fell totally flat because he doesn't really look like Picard or sound like him at all. The photo of Picard from his youth was laughable. It didn't work and it completely took me out of the story.
I'd definitely believe it if Stewart played the role, and I'm sure I'd care more about the story and even have more sympathy for Shinzon, feel a chill during the reveal scene, jaws would surely drop during Shinzon's death scene, the list goes on.
It's really hard to make an argument one way or the other though, I suppose, without actually having the alternate performance to compare, which we never will.