DC vs AC motors

In general the DC series motor will have a linear, negative slope torque-speed curve. In practice, assuming the current is limited, the torque curve will be flat out to some speed and linearly decreasing after that.

A VVVF AC Induction motor drive (depending on the control strategy used) will also have constant torque out to some "corner point" speed, after which the torque will decrease along a constant power curve to the maximum speed.