There are so many reasons for Rory to go to Bellator.
Bellator will offer him a load more cash plus he'll making extra through sponsorship. At 26, he's got plenty of years left in him and can always go back to the UFC. He doesn't know how to self promote and I can't see the UFC pushing him over Wonderboy. On top of that, for the sake of career longevity, it might be better that he spends some time at Bellator early in his career (earning a load of cash) than go through more wars like the one he had against Lawler and end up on the scrap heap at 31-32 with only the much smaller shows on offer.
Having said that if he beats Wonderboy, it is probably worth staying just for another shot at getting the belt. Without being able to self promote, it appears his only way of making serious money in the UFC is to get the belt and have a good run as champ.
Personally, I think the UFC are shafting Rory. They've offered him a contract which he's turned down so they've put him in a fight against Wonderboy, who they expect to win, in the hope of strengthening their hand in negotiations or build the Wonderboy hype of a win over Rory. I kind of hope Rory knocks out Wonderboy and signs with Bellator...