I am getting the idea that Devitt is getting pulled from all sides. WWE obviously have made their big offer, TNA apparently have made a play for him, and New Japan are still trying to keep him.
If he stuck with New Japan he'd probably get a money increase and a push for staying, but I doubt he's ever going to be IWGP champion or a top guy there, no matter what.
TNA, I see zero upside with. He'd make ok money in a badly run company that'son a downward spiral, and more importantly he'd be getting himself blacklisted from WWE (and New Japan isn't too fond of TNA either).
WWE, it's always a gamble, but if they're offering a big contract, well then even if it doesn't work out he'd make really good money for three years, then he could go back to NJPW. If it does work out, well then fuck it, he'd have a career in WWE for years and be set for life.
Seems like a no brainer to me, but I'm not him so who knows how he thinks.