Close in, slugs beat buckshot and pretty much anything in the 308 calibre. Which isn't to say 308's or buckshot can't work.
The slug you want for that close in work tends to be the heavier, harder slower slugs rather than light, fast and easily broken up.
one big gaping hole is going to do more to 'air out a bear' than a bunch of little holes.
The only 'buckshot' id consider playing with is that tri-ball stuff - 3 x 60 cal balls.. nasty business : ). But i haven't tried it yet.
A bear's cardio system is very resistant. That's far more true of a griz. (blacks' ain't so tough).
For big bears, you need to be able to break bone and keep going, and leave a big honking bleeding wound behind. Slugs do that better than buck, and leave a bigger hole than a 308.
Gad - we've been talking about this for 10 years now

I can't believe these threads still pop up.