What is pros&cons of using a unipolar stepper as bipolar?
Being as the number of turns on each pole is doubled by going to bipolar, the required current for a given torque is halved.  Thus, the torque constant is doubled and so is the Back-EMF constant. The inductance is proportional to the number of turns squared.  Thus, the inductance is quadrupled.  Thus, the break frequency of the winding (pole at s=-R/L) is halved (a lower frequency).

The effect of all of this on the performance of the complete system depends entirely on the driver!  The factors that control the stability are the effective resistance (seen looking out of the motor....back into the driver) and the level of excitation (at any particular operating frequency).

There is no effect on the step angle.  There may be some accuracy considerations.

I hope this helps...