I have read differing views on the environmental issues of cork wine
stoppers vs. plastic, or screw tops. Please shed some light on which
option I should be rooting for if I am concerned about the environment
and want to keep my opened wine fresh.

Cork is environmentally friendly.  Trees are not cut down to obtain
it.  Cork comes from the bark of the cork oak.

http://en.wikipedia.org/wiki/Cork_Oak

But anywhere from three to ten percent of corked wines may go bad,
wasting glass and wine.

The day you can get a Smith Haute Lafite in a box or screwtop is the
day I cash it in.

The purpose of a stopper in a wine bottle is to stop the contents
running out, or air seeping in before you open it. Anything that does
that and doesn't spoil the taste is OK by me.

"I am concerned about the environment and want to keep my opened wine fresh."

If you are asking about what to use after the bottle of wine is
opened, the environmental question has already been settled:  use
whatever kind a stopper came with the bottle.

If you want to choose your wine by the kind of stopper it has, well, I
guess that is a nice environmental attitude, but the better wines
still come with natural corks, whereby there are many qualities of
such, starting with the quality of the cork bark for one piece corks,
then corks made of granulated cork material.  They all have to be
treated more ways than I could imagine from what I saw in a
documentary on the subject.

Natural cork is, indeed, a renewable product, but not one in infinite
supply.  Furthermore, natural corks can harbor microbes or something
that can cause the wine to go bad, despite all the treatment in their
production (also an environmental factor, not to mention the transport
from Portugal or Spain to whereever your wine is bottled).

Natural cork is traditional and more expensive than other stoppers, so
its higher cost has to be reflected in the price of the bottle of
wine.

In Europe, there has been greatly increased use of plastic corks. 
Screw tops are still considered cheap  - for cheap wines.  Both are
absolutely adequate stoppers, better than natural cork, because they
don't present the risk of a natural cork making the wine go bad.

Personally, I wouldn't worry about the matter.  Buy the wine you like.