Ok, so I bought myself an iPod 7th gen. I now own the Ipod 5th, (just sold my 6th), own the 7th, as well as iPhone 3Gs.
With that said, I have had the opportunity to do a fair amount of A/B testing between the devices. I do not have a portable or external amp, so everything I am listening to is based on the Head Phone Out Jack.
If I had to compare these devices to one another, and rated the best device as a 10, here is how I would stack them up, thus far. I still plan to do some more testing with additional genres.
#1 - iPhone 3Gs (10)
#2 - iPod 7th gen (8)
#3 - iPod 5th gen (6)
#4 - iPod 6th gen (3)
IPhone: (Deep bass, with impact and slam in bottom LFE, excellent stereo panning, good upper detail while not harsh or strident. Overall, very well balanced.)
7th Gen: (Overall a good sounding device with good bass, but not able to deliver the same level of impact, or quite as deep, but still tight thumping bass, with a great balance of lows, mids and highs. Panning/stereo not bad, but not nearly as effective as iphone)
5th Gen: (Sweet sounding in the mids, a bit lush, too lush really, because the mids are smeared. The highs aren't bad, but not as detailed as 7th gen, does not pan like iphone, and the bass is sloppy and weak by comparison.)
6th Gen: (Thin, tinny sounding. Overall low SQ. Has numerous weaknesses but no strengths to bring to the table.)