How to choose floor insulation?
There's nothing wrong with fiberglass insulation beneath the floors.  It will be, most likely, the most cost-efficient solution.

If you have a situation where critters like the insulation, well you have a vented crawl space! Critters get in there.  

If you have an issue where the insulation gets moist and sags, the problem is not the insulation, it's a moisture/humidity/temperature issue!  If you have, especially during the summer, humidity circulate beneath the home and begin to soak into the insulation, you have a more serious problem and using a different insulation only ignores or covers it up.

Not knowing exactly where you are, putting plastic sheeting on the bottom of the joists is a potentially dangerous idea!  You are setting up a temperature difference between subfloor and bottom of joist, and then trapping moisture in that area.  A plastic 'vapor barrier' in this area can cause a substantial amount of condensation on the joists leading to wood rot.

Plastic on the soil.