depends on several factors. I know some of the older ipod classics had a pretty awesome DAC, but as far as the newer stuff I would imagine their quality has gone down a bit and they are just riding the brand recognition while just putting out "ok" products. As far as bluetooth vs ipod. It also depends on what you're playing through your ipod. if you're playing 128 kbps (or a bad recording/rip) through your ipod and 320 kbps (I don't know what google play defaults to) via bluetooth then yeah you will probably hear the difference.
If everything else is equal bluetooth is the worse signal, but whether or not your ears are discerning enough to hear the difference that's going to be for you to decide. I personally can't and if bluetooth sounds better to your ears go with bluetooth even though an SQ purist may get their pan ties in a wad.
Added: pan-ties is a filtered word?