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There are countless threads on this. General consensus though was that going to 3:15s isn't really worth it. The 3:42s will really pick up and pull hard "off the line". You may see a little less mileage though. You have to reprogram for either one.
I had a 2000 coupe A4. Cost was $500 to r/r the rear end (plus the cost of the rear pumpkin.. I think I paid about $500 for that off the forum). Made a HUGE difference in SOTP.