Great comparison. I did mine while I had the X5 - and basically 3-way tested my iPhone4, V3 and the X5. Volume matched with an SPL meter and using my DT880's. Like you - I noticed that the differences were a lot smaller than I expected, and over all impressions between the 3 were very similar - and the differences I noted were also similar to your observations.
Frequency wise I'd put the iPhone4 between the X5 and V3.
The draw for both the X5 and...