Most efficient is one 5 V 15 A power supply with heavy filtering for the motor outputs, heavy filtering for the electronics outputs, and a 5 V - 3.3 V regulator.
Mixing a 60 W of DC motors with a 0.1 W of uC is not a good idea; you should avoid it if you can. That means two 5 V power supplies, a big one for the motors and a small one for the electronics; plus a 5 V - 3.3 V regulator. The total 3.3 V current is so low that I don't think the added complexity of a switching buck regulator is going to get you much more efficiency, and it will definitely make more noise.
ak