However, I also notice even among the duplicates
(say for example one is "Piano Man.mp3" and the other is: "Piano Man.wma") one is larger than the other. I know almost diddly squat about bit rates and such... to me it indicates the larger file has a higher bitrate so I've been deleting the smaller of the two. But what if that smaller sized file happens to be a MP3? Should I still keep the higher bit rate version even it's wma? I listened to both version using Foobar2K - I honestly could not tell much of a difference. I'm running into more & more duplicates it would be nice to standardize on 1 method of deleting/culling through these dupes.
It depends on the file type used. Higher bitrates and larger file sizes are not always a guarantee of higher quality, even within codecs - over time the algorithms that they use are refined and tuned for better quality at a given file size. Across codecs, this holds true too, as well as the fact that different codecs are going to have different qualities at given sizes (and maybe for different genres of music too), and the differences are not always going to be the same - for example, one codec may be better than another at 128kbps, but the reverse is true at 256 or 320kbps.
Obviously, this does not apply to lossless codecs - they keep all the original information, so the main things you are going to be looking at are file size, compatibility, and encoding/decoding speed (for playback, for example, or rebuilding a lower-quality library that's been lost). I have CPU power galore, so the main things for me are compatibility and file size.
As for which to delete, this depends on your priorities (sorry, I appreciate this is a really roundabout way to answer the question). For me, lossless trumps lossy, always. But if you don't have much digital space (hard to believe to be honest, with storage so cheap), then lossy might be better, or a really efficient lossless codec and compatibility/encoding speed be damned.