What jimbouk said, really.
I've not used the P990 (I've tried it briefly from colleagues and friends who have it, but not really used it, so I can't judge it), but I had a P800 and a P900. I decided to go with the M3100 when I moved to Orange a couple of months ago. For someone who's used to Symbian it takes perhaps a couple of days to get used to and learn where certain things are and how to do certain things. I am very happy with the choice I did, but I know people who are equally happy with the P990.
The M3100 is also perhaps slightly less a "phone" in that there is no number pad like the P990 has. There are shortcuts to get to your contacts, your recent calls, your speed dials and so on, but no one-hand way of directly typing a number. For me that was not a big thing as I had already got used to the P900 without the flip (it broke around a year ago and I never bothered having it replaced). Actually, technically that's a lie - the P900 has a "virtual flip" (which I used), and the M3100 has a similar thing, so you can use the fingers to tap number directly on the screen.
Both devices are excellent and offer almost exactly the same set of features, so at the end of the day it mostly boils down to personal preference. If you have not used either type of OS before (Windows Mobile and Symbian), then you should perhaps ask in a shop if you could try both handsets to try get a feel for them.
Edited by Incarniac, 17 December 2006 - 01:28 AM.