IMO surviving in a fight against a grizzly would be easier than surviving a fight with a Siberian tiger. The main factor on my decision would be the speed of the animals, as tigers would be agile and aim for vital points without a moments hesitation, where as grizzlies are known to have slower attacks (but still a bit faster than what a normal human can dodge at) and powerful blows, but charge and maul at you wildly attacking any body part they can reach. As well, another factor that bears are probably easier to run away from is that bears are omnivores and humans aren't usually on their natural diet, where as tigers are known to hunt and eat people for food. Also there has been cases where people just acted dead in front of grizzlies and the grizzlies ignored them. So I would choose fighting a wild grizzly, as if you are skilled enough there might be a chance of defeating a grizzly by either out smarting it or training your self to dodge fast attacks then go attack vital points.

This is like asking would you rather fight a buff and skilled roman gladiator or fight a skilled ninja.