I can't get into technical breakdown like matonga here, but it's not exactly about mp3 decoding of iTunes or Foobar.
What Heril said does stand true, but let me explain why foobar is better than iTunes. It's not the decoding itself.. foobar let you use either ASIO or Kernel Streaming, bypassing windows audio mixer, thus removing one processing step from audio transfer path. Each step added to audio transfer affects the output. So when you're using Kernel Streaming or ASIO, your audio is preserved from the player's decoder to audio hardware.
Besides that, the end quality ultimately depends on the source audio.. and that's where heril is correct. If you're going to play a lossless wav file on crappy hardware (speakers, amp or sound cards.. etc.) the final output isn't sound that good. Similarly, playing lossy audio like mp3 on good hardware will bring forth the lossy areas of source audio.
The whole point of using Foobar output in iTunes was to use Kernel Streaming or ASIO. If you're using Foobar without kernel streaming then it doesn't make much difference. Both Heril and jx12345 are right to certain degree. But then again, that doesn't anyone a right to throw insults or use harsh language.
Ultimately, the sound quality is again... subjective for person to person. Like matonga said very correctly, some like using DSPs and enhancers while some prefer to listen to purest form of source file. For example, I prefer higher bitrate MP3s or Lossless files while listening on desktop, but I burn cds for my car in nothing more than 128kbps mp3s. Because while driving I just want to listen to songs, quality doesn't matter as there are other audio interefences in surroudings. It's not like I'm going to pick up a subtle note very clearly in car.