well, she scales harder, is faster, has a longer ranged gapcloser, benefits from tank items better and has better CC.
She is absolutely not faster in the jungle. She's slow as hell and gets beat up. New Gragas can Q(to throw)-prep W-E-AA(with W damage boost) and Q(explode) and
Bam dead camp save the big monster who'll only need a few AA - and this is starting at level 4. With Spirit Stone he can get this full rotation off at every camp with only a small hit to his mana pool. Sejuani is a depressing joke vs. jungle camps.
I disagree with better CC as well. It's all trade-offs, tied at best. Sejuani slow is stronger, but New Gragas can apply his at range. Knock-up vs. Stun on first target hit makes little difference. As far as ults, depends on teamcomp. AoE team, Sej is better. Pick-off/Assassin comp, New Gragas is better. Sej ult is better at engage, Gragas ult better at disengage.
Scale harder? That would be nice if she scaled hard like Amumu or even Zac, but her scaling isn't good enough to justify that deadful early game. Benefits from tank items better? Are you referring to that 10% bonus health on her W? At 4000 health, she gains ~200 damage over 4 seconds. That's nothing to brag about. Otherwise a full tank New Gragas will get just as much benefit out of that sort of build.