The D80 will be the most modern of your choices, the D70s is a bit old hat now, things have moved on a bit since it was launched, the D80 is the only camera in your list which is still current and can use virtually every lens Nikon ever made, canon cameras need EF or Ef-s lenses so your choice is a bit more limited.
I've had a 20D and a 30D and sold them both and gone back to Nikon first with a D200 then a D300, have you considered the D200, they can be had for under £400 secondhand now.