Well.... let's not talk about what camera to get at this point. What lenses do you already have? The camera's honestly the last thing to think about when you have to spend almost twice as much on lenses to get your job done.
On a side note, I've been using the D1x/h, 2h/Xs and also the 1d series for paper work for the past 5 years, and the D3 is a camera that's been hitting the press market hard, forcing lots of sports/fashion publications to consider hard against switching back again from the other camp.
Here Tokyo, the D3's seen quite a bit of use in the professional field, with most of the tabloids and magazines switching back from the 1 series of cameras to the D3, using lenses they've kept from the days of the F4s/F3. Personally, i staff part time for a number of publication, and the D3, to me, is something that's giving canon a run for its money in terms of performance, price and functionality.
Ok.. I shoot canon most of the time due to work (issued equipment, etc etc). The 1 series is great for everything under the sun yes... however lens performance, particularly in the wide range, tends to bog our workflow down. If you want something cheaper, consider the 5d which is a great camera too, though i've run through 2 bodies in the past year. lenses wise, i tend to go for nikon as they just last much longer, and feel better. (Refer to camera quest Nikon/EOS adapters)
What's your shooting style? what lenses/system do you already have? What do you want to do with your equipment? For the M8, read through the M8 forums, the rants/raves, praises/lusts over it, etc. Get a clearer picture on what you want your $5000 to get you. For the D3, check sportsshooters or nikonians.org for reviews/field use/tests, etc. If you have any further questions, feel free to PM me. I'll be receiving my D3 next week for an assignment and could provide some info on it.