Off-Roading in an S4 or A4? Those are 4-door sedans. One is a turbo V8 made for performance while the other is a nice road sedan made for spirited driving. But off-road?? At least the Q5 has some ground clearance and has larger tires that could withstand some bumpy roads. Now, if you were a geologist who wanted to get to your destination before eveyone else, get the S4, except for the fact that you'll need to stop for gas more often. It does not get the best gas mileage. But neither does the A4 or Q5.
Which one is better for a geologist? I believe that includes some off roading as well.