Alot to do with the coating, some of the taps for stainless and other exotics have special grinds to the flutes that make them cut easier or break chips etc.
For aluminum I pretty much now only use form taps in my shop, Balax makes some good ones (threadfloer taps) as well as OSG.
http://www.balax.com/ (Balax even makes a form tap that creates a locking thread in the part) Have used form taps in steel as well with great success, just don't have many form taps for steel yet. Biggest 2 advantages of the form taps are; no chips, and probably 5-6x tap life. I have a 1/4-20 form tap that has been used for 1" deep through holes in about 5,000+ holes and it still looks new.
Cut taps in aluminum, bright finish no coating works well, other wise ZirconiumNitride.
TIN coating works well in Steels as well as steam oxide with oil.
If buying new taps for steel I highly recommend getting spiral point taps for through holes (most of the small taps are this way, sometimes called gun taps). This spiral point pushes the chip out ahead of the tap and keeps from binding in the flutes. When using these types of taps you will notice how much easier they turn, and you don't need to keep backing the tap off to break the chips just turn till your through in one shot.
If doing blind bottom holes, the spiral flute taps work great at pulling the chips out just like a drill bit. Then you don't wind up packing the chips in the bottom of the hole etc.