As someone who - still - prefers film I would ask why do you
need digital?
When you refer to 'moving up' do you mean from the M3 or the D20?
If there is a need, or you simply prefer digital then fine, and the M8 is the obvious choice. Assuming you keep the D20 you have a digital solution already, but again, if you want the M3 kind of experience in digital form.............
I can see the advantages of an M7 but if buying new, I think I would be inclined to buy an MP - basically the same camera bar the viewfinder, as your M3. I have an M2 which is perfectly fine. Buy an MP and you have a camera for life (don't worry about being able to buy film!!!).
Someone was looking at my M2 when I was shooting with it last weekend. They couldn't beleive it when I told them that yes, it was an old camera, but you could buy pretty much the same camera now brand new, for £2K! I'm not sure if they were more surprised by the cost or the fact that you CAN still buy film cameras
In fact, what you really need to do is buy and MP and an M8.