You can only compare quality of video or audio playback of players if
both will play the same video or audio in some format so that quality
can be compared directly with the media file thus not being a
variable. In short, the players you mention and several others can
play some media formats well, but none play all formats well or at
all. Actually Real 11 plays many more formats than WMP, as they are
delivered, but several plugins can be downloaded to extend what WMP
will play. Real 11 will play most Microsoft formats. Microsoft will
not play Real formats, although there are a few underground plugins
that will make some versions of WMP play some Real media formats. WMP
has the most elaborate light show, but the Real 11 one is not bad.
Library functions, if you use them, are somewhat different on
different players. If you want to play most media formats around with
convenience, you need several players. I have WMP 11, Real 11, QT,
Winamp, SWF, and a few other special purpose players all installed on
my XP at once. Since some common formats such as Microsoft audio and
video formats are played by several players, you need to carefully set
up what formats the player is to play. Always reject the offer by a
player to be primary for everything it will play when you download it,
or you may find your Microsoft formats playing on the Real player,
etc.

You can download either in free or pay versions. Make sure you select
the free version when you download, if that is what you want. Pass up
any offers for free trials of premium services. You may have to
carefully cancel these free services in a certain way to prevent a
charge for them starting automatically at the end of the free period.
In other words, read all of the small print. I believe Apple now only
provides a free download of the QT player combined with a download of
the iTunes player. You could of course dump the iTunes player later if
you don't want it. The QT/iTunes player combo download is very large,
and I hope you are not on dialup if you take it.