I assume your referring to a 6 or 8 wire motor that can be run either as unipolar or bipolar. Bottom line there isn't a clear winner. Facts:
Running bipolar is stonger, rule of thumb 25%.
No difference in step size as long as you comparing equavalent steps, full step to full step, half step to half step.
If you run the same motor as bipolar then run it as unipolar with the same power supply, the bipolar wins hands down.
Sounds like bipolar is the winner, but it isn't that simple. True performance is the total system which includes cost, speed and power. Performance with all factors make it less of a clear cut. Unipolar motors are simpler to drive and cost of drivers usually reflect that. 4 wire bipolar motors are slightly simpler to build offsetting some of that. Due to the switching arrangement implenting unipolar vs bipolar, at less expense it is easier to run higher power supply voltages. Running higher power supply voltages allow higher step rates. So you could argue, that at the cost of implementation of bipolar, you could spend more money thus getting larger unipolar motors. So it starts to even out. If you already have 6 or 8 wire steppers, run them bipolar if you need max power. I have found it hard to find larger torque motors in unipolar. (400 oz in and larger)
Here is an excellent source on steppers:
http://www.cs.uiowa.edu/~jones/step/