I have a Onmia i900. I'm really happy with it for many reasons. GPS on this thing is awesome, nice big touch screen is like having a dedicated GPS unit. Its camera takes good outdoor shots which I think are better than most 5mp cameras out there. Since running firmware H1 (mid August update) video quality improved a bit to about 20fps now. Its still crap when compared to a N95 or a LG viewty or Secret. But its acceptable.
I can type pretty fast using the Samsung keyboard in portrait mode, lots of space for the keys. It took a bit to get use to after my N95 8gb but I'm loving it now.
And often I watch movies and its great on that screen.
Reason why I got it over the i8510 were, it was out before *main reason*
Its slim and lighter, GPS is great. A Touch screen is nice.
Heres some shots I took using GPS. I use Garmin XT or Tomtom 7


Sample test photos I took using the camera (5mp)




As you can see for a 5mp camera its pretty good.
Its really depends on what you want.
Go for the Omnia if you want:A touch screen
nice for GPS
Watching Movies on a bigger screen.
Hand writing recognition
Windows mobile 6.1
Get a i8510 if you want:Normal keypad
Best camera in a mobile to date
better video recording.