I prefer he stay in WWE, honestly. Short of the injuries, and him asking for time off, he was a champion and can now be used in the main event, and to put over people - which, at this stage of his career, is probably the best for him. He's a great wrestler, yes, but it does nothing for him, imo, to decide 'Hey, let's bus him back to TNA, the number 2 fed /in america/, for him to be 'used right', because let's face it, Edge was better, and the best Christian can do is being top champ in TNA.' That's some self-value killing type of stuff.
That's, in my view, saying - Edge was better than you, couldn't make it to the top in the number one fed, so cut your loss demote yourself back to TNA and be a champion there. It won't help him escape Edge, who was both top champs(WWE/WHC), in WWE, and the best Christian could do was be a champ in TNA to try and 'escape' it.
Basically, him staying in WWE would be best at this stage in his career- they, apparently, give him time off when he wants it; they've made him WHC twice(both, shitty reigns), gave him multiple shots, great matches(With Orton, and almost everyone else he's faced), he gets paid well(Most likely), and is likely financially set for the rest of his life, if he doesn't squander it away, and can probably get a job backstage with WWE once he retires from in-ring competition.