As a Cal fan that has been to both Autzen and Neyland...
I was in Neyland in 06 for the Cal game. There was a lot of pent-up angst from a disappointing prior year, and I don't think the locals were happy about this west coast pac 10 team (known for its liberal/hippy history) rolling in ranked higher than us and favored by a point. Without a doubt, as the Tennessee players came out of the T and entered the field... that was the loudest moment in any stadium I'm ever been to (and I'll be surprised if it is surpassed period). Tennessee fans in attendance said it was louder than any recent game to that point, perhaps only rivaled by some late 90s UT/UF games. And the roar/shaking upper deck when they scored their first TD still brings goosebumps.
I was also in Autzen in 2007. Peak noise did not seem as high, but the stadium is very, very loud. Most stadiums have a kind of linear 'build-up' in noise as their team is on defense that crescendo's towards its peak right before the snap. Autzen's does that too but it doesn't seem linear... it rises kinda exponentially and continues rising to a noise level beyond what you think a stadium of its size has any right to be.
Its worth noting that the Ore-Cal 2007 game was close throughout (though the 1st half was a miserably played football game... not so for the 2nd), while the UT/Cal game was over in the 3rd quarter.
One other difference... the fans. When I made the trip, i was a student, broke at the time, and made the trip for $50 total (slept against a UT library the 1st night). Several of my friends flaked but I was determined, and the trip was made possible because a Vol fan offered to drive me from Nashville to Knoxville on his way. He even put me up in a hotel the second night cause he felt bad the way the game unfolded. Also, TNFlower over on the UT scout forum (maybe here too) invited the entire Cal message board over for bbq... and IIRC she, her family and friends made ~200 lbs of southern bbq (something ludicrous like that) without asking a dime in return. Moreover, we met locals who offered to trade us jars of moonshine for Cal t-shirts (which we'd drink with them in the parking lots as we got it) and just in general... hospitality was off the charts.
Oregon fans are devoted, but we got waaaay more snide comments and pissed off replies after the game... though again, that may have been a byproduct of how the game unfolded. Not much in the way of 'southern hospitality', but I guess you can't expect that everywhere (though Cal fans made a concerted effort to return the favor to UT in 07).
I told everyone on the Oregon board that they needed to make the trip out to UT. Its a special place to watch a football game, thats for sure.