ORIGINAL: ShatoDavis
Its the age old lighter/faster vs. heavier/slower debate. I myself opt for the heavier/slower side myself. That being said I would say that the 300 ultra would certainly do the job. Everybody is getting their panties bunched on this thing, assuming that the guide was ordering the customer to buy a new rifle. I seriously doubt thats how it was said. I would say that he simply stated his preference. As said before, people go brown bear hunting every year with a bow.
I would say that you take a rifle that you are comfortable with. If you are comfortable taking the 338 great your guide will be happy. If not, then take the ultra as I'm quite certain that it will do the job. Your guide will cope I'm quite certain.
I thought of it more as an issue of whether the guide knows his ass from a hole in the ground or not. If your guide thinks that you have to have a minimum of a .338 magnum to kill a brown bear rather than a well handled .308, FIND A NEW GUIDE! You're paying a LOT of money for their expertise, you better make sure you're getting what you pay for.