Man, I would LOVE to see either of these guys guest host an episode of Raw, especially for the head to head "war" with TNA on Jan. 04. In a perfect world, I'd like to see both guys (different nights, not on the same episode), but I fear in actuality we are going to see neither.
If I had to choose one of them, especially for the Jan 04 episode, I would pick the Hitman. Let's face it, neither guy is going to get much involved in the action physically (Bret because by all accounts he's not physically able to do so, and the Rock because he likely won't want to and risk an injury which could affect his movie-making status). This would be particularly effective if this could be done and kept quiet, ie, not announced ahead of time as per usual, and not spoiled by the Internet/IWC. Because if the WWE announced a surprise guest host for this night, many of the guys who were planning to watch TNA that night would have to tune in to RAW to see who the guest host is, and if it's Bret, with all of the potential this could hold, they wouldn't be returning to Spike on this night. And when those who plan to channel surf between the two find out it's Hart (hypothetically), it'd be Hulk Who?
I cannot see the Rock doing much more than a one-time shot, so while a guest hosting appearance by Dwayne would be huge, I don't see it going anywhere. He'll not get involved in the action, he'll bring his tremendous charisma to the one show, then he'll return to Hollywood and continue to forsake his WWE past.
But with Bret Hart, there's potential for ongoing action beyond a one night appearance. There's interaction between himself and Vince and HBK, bringing closure to the Screwjob at last. This could contribute to a turn by HHH, or even HBK himself, setting up a feud between HHH/HBK at WM26. He could become GM for the Raw brand. He could mentor/manage the Hart Dynasty. It has been speculated on this site previously that Hart was open to returning to WWE in some capacity; this could eminate from a guest host appearance on Jan 04, something that a Rock appearance probably doesn't bring.
And food for thought, what if Bret's physical status is better than we all think? Before anyone reminds me, I know all about his stroke and all that, but I'm not convinced he could not perform to a very limited degree in the ring. I'm not suggesting a full time return to wrestling in a full time capacity, but I wonder about match interference or even a one-time match with someone. I saw Bret recently and I didn't think he looked too bad. I remember years ago, when HBK was retired and I wondered to my friends if he might ever return, and I was mocked and laughed out of the room, being told his back was so messed up that you'd never see him back. When he did return, the same people told me he'd only go to a limited degree and he wouldn't be effective because he wasn't able to go, that he was one bump away from major trouble. And who was it that just received a Slammy for match of the year in 2009, over a decade later? Food for thought.