When doing a "Mix or Render" in Metro, select the following in the related dialog box:
Type: To File
Sample Rate: 44.1kHz
Sample Bits: 16
Packet Size: 128
Quality: Best
Then click the "Mix" button.
Raising the Sample Rate or Bit Depth during mixdown will not improve your audio if you originally recorded at 16/44.1.
When you're done, try playing the resulting AIFF file in the Quicktime player instead of iTunes. If it sounds fine in Quicktime, make sure you do not have the iTunes equalizer set to anything other than "flat." Also, be sure that iTunes is playing the AIFF, not a converted MP3 or other file format.
Regarding Export/Mix, Export creates an audio file from a selected track only. Mix creates a file from all selected tracks and includes mixer settings, plugins, aux busses, etc (your mix).
Recording at higher bit/sample rates can improve the dynamic range of your recorded tracks, but at the expense of larger file sizes and reduced computer performance. But before jumping to higher bit/sample rates, I would work towards refining your recording and engineering techniques. Better micing and mixing practices can have a much greater impact on the finished product than simply increasing resolution. Also, you want to keep in mind that reducing your mix down to the current CD spec. (16/44.1.) can create artifacts if you're not careful.
Mike