Teenagers might appreciate the Mazda 3 more because it's smaller and cuter. Lot more fun to drive than the larger 6. altho' having a bigger car would mean you could haul around more of your friends, and it'd be little safer too, since it's a bigger car. Might have little bigger engine, too, for tiny bit of more power, but don't expect great power, unless it's got a 6 cylindar in it. If it's got a 6 cyl, it might have lots of power for a first car, but then from a safety standpoint, it would have enough power to get you out of the way if necessary. So yes, a 6 cyl would let you drive lots faster, but also have enough power to help you if really needed.
You might consider the costs of insurance between the 2 cars. One might give a better benefit since it's going to be for a teenager, even tho' someone else might be paying for the insurance.
