Metal vs. plastic tire levers What are the advantages of buying plastic over metal tire levers? (or vice versa). 

Which ones are easier to use?
It largely depends on the usage you want to make of them and the particular tire/rim combination you use.

Plastic levers are usually small and lightweight, and if they are of good quality, they are enough for most tires that are properly fitted to the rim. I mention this because sometimes, rims and tires can physically differ from the nominal size they bear in their labels, so a tire may fit way too tightly in a rim. Other times, the wire collar in the bead may be too stiff. These two situations make them very hard to pry out, thus a tiny plastic lever may break, however, this is a rather rare case.

Small and light tire levers are ideal to carry in the emergency tool set for on the road or on the trail repairs. And for most cyclists that may now and then perform a tire swap at home.

Metal tire levers may be heavier or more expensive (due to being manufactured from fancy metals), and usually are bigger and better suited to shops where several tires are swapped or repaired by the same user each day. In this situation the large lever makes the work easier by providing (you guessed it) more leverage. They are also well suited for the situation described above, specially if for any reason you are forced to almost always use a tire that "almost fits".

Another reason to prefer plastic levers is that it is very difficult to damage a rim or a tire bead, while a poorly polished metal tire lever, or a poorly used one may easily damage the rim or the tire.

In the other hand, I would advise not using plastic levers that are too flexible or too thin and brittle.

Keep in mind that there are also levers made of a metal core covered in plastic. They are kind of in the middle of both sides.

Note: I consider tire levers survival equipment, along with patches and glue and/or an extra tube, thus, if first time buying tire levers, or first time buying that brand/model, I strongly advise testing them at home before engaging on a long tour alone or you being the only one carrying emergency items like these, so you don't get stranded in a far place with a puncture and a pair of failed tire levers...