I'm looking into getting a track light unit that can be shaped up to a point. Each unit has 5 of those little spot light type bulbs. The less expensive one is mostly of plastic while the more expensive one is mostly metal. From a safety and longevity standpoint, which is better in the long run?

The major factor in safety and longevity is how much use/abuse they will have to take.

If they are mounted high on a ceiling, where nobody bumps into them, and where they are not adjusted often, just set and left alone, any of them should work fine. Go with the cheapest ones, probably plastic.

If they are frequently adjusted, or are low enough that they get bumped into occasionally, spend a bit more to get sturdy ones, probably metal.