FLAC vs Mp3 quality - 29th Sept

Recently, I've got audio files of the System of A Down: Mesmerize album in FLAC (Free Lossless Audio Codec) format which suppposedly is of higher quality than the common Mp3 format. I've also got the same album in the Mp3 format. In which, I compare a couple of songs alternatively, meaning that, after listening to the mp3 version, I would then listen to the FLAC version to compare the quality of both versions. In terms of bitrate, the FLAC version has undoubtedly higher value, and the file size is approximately double of the mp3 version. After several listening sessions, my verdict:- I couldn't hear even slightest difference?

Conditions:-
Platform = Desktop Win XP, Winamp 5.50
Headphones = Philips (Don't remember the model)
Songs:- Questions, Lost in Hollywood,
Soldier Side (Intro), Radio / Video

And I thought, maybe there are a couple of factors leading to this:-
1> I don't have trained hearing or that my hearing isn't that sensitive.
2> The audio port isn't plugged in directly into the back of the computer but to the mini-compo which is connected to the desktop audio out.
3> Winamp is not a good player for audio playing?
4> The platform used is not suitable to test audio files.
5> The headphone isn't that high quality.

I wonder if any of these factors have caused me to have not detect much difference? With the smaller file size and if my ears cant detect the difference, I would say that I might as well use the mp3 format for the music. Furthermore, mp3 is more universally accepted while not all players / phones support the FLAC format. Perhaps someone else has other opinions on this on how to improve my music listening experience?