Kinda like fantasy hunting--

What if etc--

But--
If I had to choose between them I would choose the Grizz.
The Grizz is heavier boned and just as quick with a long paw/reach with razor sharp claws vs lion teeth. One stroke would crush a lion in mid jump.I doubt the lion could tolerate the damage and continue the attack.Could a lion get and stay on a bears back? I doubt it.One bear bite or hit and its over.

Seeing a big bear at full speed is a scary sight indeed.

My money would be on the Grizz.