The Nokia 5800 XpressMusic, codenamed "Tube" (as it is more formerly known) is designed for two things - music and video - and I'll save you the bother of drawing this out by saying simply... yes, in not just my experience, it is better than the iPhone.
Forget personal preference in this because that will always get people unnecessarily angry. The screen is bigger, the video quality is better, albeit it is dependent on the file itself, but the high-definition screen will make things clearly noticeable. It's lighter, faster and far cheaper (again, depending on network).
And the music quality, my God. It's never easy comparing the sound quality from one device to another because of the multiple variables; the hardware, the software, the headphones and the bitrate of the files.
So, I ripped an MP3 from a CD at the highest bitrate and the best quality possible and transferred it to the memory card on both the 5800 and the iPhone. I even set the equaliser the same on both and configured the settings as identically as I could.
The 5800 had the better sound, but only slightly; the iPhone had a touch of echo and a metallic sound, if that makes sense.
Let's just take a quick look at the differences between the two phones, and you'll find it clear as to which one I think is better.
(Original from here, modified and edited slightly).