I can't agree with MNguyen.
Sound quality depends on:
1) The phone/player (hardware)
2) The output (headphone/speaker)
I have N93 now. I never had/try the N91 headphone audio quality.
But i heard that it's the BEST.
Having own N93 for about 3 months, i'm VERY satisfied.
I used sony disk-man a long time ago; Ever tried I-pod before.
In my opinion, N93 sound quality (using the standard nokia headphone) is comparable to I-Pod.
I don't really know about N91 but from what i've heard, it's the best for music. (or probably it's the headphone difference)
My suggestion is,
get Nokia N93 ALONE!
You'll be satisfied with it.
wi-fi, 3.2mpix, VGA recording, new and fast processor, above average sound quality, what else do you want?
Otherwise, wait for N95.