First: The selection of a Uni-Polar or a Bi-Polar stepper motors may be determined by the stepper drivers that you intend to use on your system.
That said, Bi-Polar stepper motors are more efficient than Uni-Polar.
The difference between the two types are:
Uni-Polar are designed for the +motor voltage to be applied to the motor winding centertap on each phase. The Uni-Polar driver then switches the 2 end wires to voltage common in a pre-programmed sequence. As a result, 6 wires are required to operate each motor.
Now to confuse you further, a Uni-polar stepper can be wired in Bi-Polar mode by insulating the center tap wire and not use it. You would use the two wires on the first winding as A and B. The two windings on the second winding would be C and D. If the motor runs in the wrong direction on your machine, just reverse the wires on one of the windings.
The advantage of Bi-Polar over uni-polar is that it gives you more options without stressing the stepper motor itself. A Bi-polar stepper normally has 8 wires. The internal wiring is almost the same, but each half winding is brought out on a different color coded wire. These windings can be wired in series (like the Bi-polar -not using the center wire), or in parallel (top wire on each winding connected togather, bottom wire on each winding connected togather).
Uni-polar is older technology (I think) than Bi-Polar. I personally would not use a Uni-polar driver. There are too many excellent used Bi-Polar drivers (at $50.00 or less each) on e-bay and other sources, they are almost fool proof to wire up, and seemingly last forever. I have built about 4 different drive systems using used drivers and have yet purchased a defective one.
If you need some used drives and motors (Bi-Polar) with guarantee of not DOA, I know a guy in Idaho that I can put you in touch with.
Hope this helps.