OMNIA
1. Omnia screen is smaller.
2. Omnia screen is lower resolution. ( have you seen the two screens next to each other playing the same video, i struggle to call the iphone clearer)
3. Omnia runs Windows, which is less stable. ( if you want to use cooked roms its great
mine is currently using the HTC software.)
4. Omnia less memory on the base unit 8G, but expandable via MicroSD card. ( comes standard with 16GB with the 8 extra being a memory card.)
5. Omnia uses touch screen as well as a stylus. (yup)
6. Omnia battery is replaceable and you can carry spares. (if you need)
7. Omnia battery life is much the same as iPhone.
8. Omnia can be repaired quicker and cheaply.
9. Omnia has smaller user base. ( might be smaller but its not small, omnias sold like hot cakes

)
10. Omnia more likely to be replaced by something else than supported in future. ( i think the i910 or something is coming out later this year... which will be 1 better)
11. Omnia GPS has GARMIN maps for life. (Turn by turn?) ( yup its turn by turn, its the garmin software, so works identical to a Garmin in the car)
12. Omnia has 5MP camera.
13. Omnia can take video clips. ( in slow motion if you want aswell

)
14. Omnia handles various types of media well. ( except for .mkv files

)
15. Omnia has more Office software readers available.
16. Omnia has more free software.