Tend to agree with the previous post in terms of the specific DAC and its implementation being extremely important. That said, it also depends on how much value the OP places on the convergence aspect - he may not want to use a separate device as his music player and might want it all in one and be prepared to compromise on pure sound quality for that. Things get a little trickier then I think.
Firstly, as per the above post don't worry about the player software. In Android there are many alternatives so you'll find one that you are happy with, in WP I don't know about alternatives but I expect they are there and in any case the Zune player should be competent (although may be lacking things like FLAC and ALAC support which may matter to you).
In relation to the DACs in the phones - are you getting a US variant S3, or the international one? The US ones use the S4 Krait SoC which comes with a specific DAC onboard, the Odyssey will have the same DAC since WP8 will be utilising the S4 architecture too. That said, the implementation could make the same DAC sound better or worse and there is no way to know that without ears-on (good example being the Galaxy S, average SQ on stock ROM despite a nice Wolfson inside, very nice SQ with Voodoo Sound a tweak software that requires Root access). If you're getting the international S3 then the comparison is Wolfson to the S4 DAC. Not seen much in the way of comparison there, but what I have seen seems to suggest that the S4 DAC is reasonably competent but probably not on a par with the Wolfson (and also will never have Voodoo Sound support, which is meant to be coming to the international S3). As it happens I have an international S3, and will also be getting an AT&T S3 next week (by the looks of tracking information to date at any rate), so will be able to at least provide my own subjective impressions of the two then.
One thing worth mentioning about the S3 is that it works with USB DACs, so you have the option of upgrading the sound as your need and budget dictate. Very unlikely to have this facility available on the WP side of things. You can also improve the SQ with custom kernels and software mods too, so even if you didn't want to go down the line of USB DACs you have more tweakability at your disposal where the sound is concerned than WP will afford you. That will either be something you enjoy doing, or an exercise in frustration, depending on the sort of person you are.
I've been fairly happy using mine with a FiiO E7, and as of next week (providing it actually arrives), I should be using a HeadAmp Pico USB DAC/amp with it (into Beyerdynamic T5p, providing it arrives also). It seems like USB audio is going to be much more ubiquitous in Android going into the future, so like the situation the previous poster has with a seperate player this means you can then simply ignore whatever DAC the manufacturer puts into their phone and know you'll be happy from the audio point of view.
Finally, worth pointing out that by the time the Odyssey comes out you could have been enjoying your S3 for months and months, given how stellar a device it is all-round that is worth factoring into your decision.
Just some more food for thought.
Edited by NZtechfreak - 8/9/12 at 5:56pm