I would think that without +buy, the Duke strategy will beat out the Silk Road strategy, since each can only get one victory point card per turn. 3 Duchys and 3 Dukes are 18 points whereas 3 Duchys and 3 Silk Roads are 10 points.
I think the difficulty lies (and perhaps this is your question and I oversimplified it) in deciding what to do with $4 and $3 hands - do you get silver to support your Duke strategy? When do you get Silk Roads on $4 (that is, with how many Dukes left)? When do you start picking up Estates with <$4?
Now that I just wrote that, I am pretty sure that was the OP question, and I just mouthed off before really figuring out your question.