It depends actually on how IW wants to use him. If they plan on giving him more titles instead of making him lose to Dreamer on PPV (
) then he would be much more useful in TNA than in WWE.
From a roster standpoint, assuming AJ would make the move he would be the perfect man to take Jerichos spot if Jericho doesn't return, he's been branded as the face of TNA but people who watch the product know that he is more in the CM Punk and Jericho spot and doing his job as good as those two.
On the other hand, whenever he is facing a good wrestler the chances of the match beeing below **** is pretty slim. As far as I'm concerned only Daniel Bryan has a workrate in mainstream wrestling like that (that is if WWE creative weren't afraid to put him back on PPV, it ain't his fault his outshining the rest of the roster, nothing to be ashamed about ).
In the last 12 months or more he has mostly been booked against mediocre to bad wrestlers (Pope, Abyss, Bully Ray, Tommy Dreamer, RVD off the top of my head) but he still hasn't had a bad match and the storylines involving AJ are surprisingly good by TNA standards most of the time ( although there sure are cases where this statement doesn't apply, see him losing to Dreamer on PPV again lol ).
Now if Jericho returned you'd literally have three guys that can make any storyline work and make most guys look good, from an entertainment standpoint that would be great obviously. Still it comes across as a bad balance between the WWE and TNA talent pool. Noone in IW is ready to take his spot so it would probably do more harm to IW than good to WWE switching brands while like I said WWE already has one guy (Punk) in that spot with a chance of a 2nd one ( Jericho ) returning.