What is the best buy?? 3.6 or 4.2

Im looking into buying a Audi V8... with the money i have i can either buy a nice and ok 3.6 automatic, or a hi-mileage not so ok 4.2... so i was wondering what model to buy?? its 100pct sure the car will be tuned up with either compressor or twin turbo so im also looking for the strongest engine... hope some one can help.. best regards