Which is more powerful?

The motors power is in direct correlation to the batteries feeding it. If you are using good lipos then as stated above on 4s it will be comparable in top speed and out accelerate it every time. on 6s lipo the picco stands no chance what so ever, but the vehicle will be severely overpowered and a beast to drive and keep it going the way you want it to.

4s at a 125 amp draw will make 1850 watts (2.5 HP) and 4.06 ft-lbs. 125 is an average as it will peak up to 150+ when you peg it hard. A spike to 150 will make 2750 watts (3.7 HP) and 4.87 ft-lbs.

6s at a 125 amp draw will make 2750 watts (3.7 HP) and 4.06 ft-lbs. A spike to 150 will make 3300 watts (4.4 HP) and 4.87 ft-lbs.

the spikes to 150+ are just that, spikes. Average driving should pull 125 amps or under, but yes it is quite capable of handing that big block picco it's ass.