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 V3 (for me) is being able to drive full sized cans without an amp.
Anyway - great job.