Scanner or D20

[MatthewCromer]

Your 6x7 should beat just about any dSLR other than the 1Ds Mark II assuming the best primes on the Canon.  At ISO 400 the 20D prints should be very respectable compared to the 6x7 in terms of noise/grain but with substantially less total resolution.

With ISO 100 transparency film and a decent scanner, the Mamiya will eat your 20D for lunch.

[Ray]

Matthew,
You missed part of the question, which was, "

I do A3+ prints, never larger than that. Is D20 good enough? What to do....?

The answer is clearly, yes.