Andy -
I'm a chemist by training, and work with electronics and optics regularly. But I'm not a graphic artist, professional photographer, nor optical expert. Here's my opinions:
Reading the Tamron link Jason provided, I conclude that the so-called "digital lenses" are simply lenses designed using digital technology - i.e., computer programs. As have virtually all lenses for the past twenty-thirty years. Maybe more sophisticated programs, but there's really nothing absolutely unique about the lens making it superior to a non-"digital lens", at least insofar as I can glean from the info on that site.
Optically, you may find the Canon lenses somewhat better than the Tamron in practice. While I've read that there are differences in tone and in limiting resolution, one rarely shoots Macbeth checkers and resolution targets. I suspect that the availability of the USM on the Canon is probably a more important factor than the actual resolution, given the high quality of either.
The angle of the light and the brightness of the image sounds more like a lens coatings issue than anything else.
As for color tone and CMOS vs. CCD - well, the relative sensitivity of the sensor to light of different colors IS different for different technologies. But, given the color-adjusting abilities of digital technologies, I don't think this should be the deciding factor.
In short, if you've got a real investment in Nikon glass, don't switch to Canon thinking the glass is better. On the hand, I am given to understand that many people consider the Canon software superior to Nikon's, both in terms of digital imaging AND - and IMO more importantly - in terms of focusing and exposure determination.
Good luck, sir.
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