CMark: Why do you assume that TaxNerd is renting a car? Did I miss something here?
And, you say that the "407 only saves you from downtown Toronto" Not so! It "saves" you from a lot of heavy traffic along the QEW West of To all the way to Burlington! I am not familiar w/ the part of the 407 East of Toronto but it looks like there's a major saving there too! I think 407 is well worth the toll! It's a much more enjoyable experience than the QEW any time of day IMHO not just "during rush hour"!

Ok folks, I just have to throw in my 2 cents worth.
Many posters have given good responses, but I agree with BAK. I drive for a living and am in and out of Toronto daily, sometimes many times in the same day.
BUT just for the record, taking the 403 down from the 401 to the QEW is actually longer than the Don Valley Parkway. And it's actually longer than the 427.

The 401 , DVP, Gardner, QEW is actually within a Km of being the same as 401, 427, QEW.
We never take the longer 403 unless we hear of a bad accident elsewhere.

To the original poster, tune your radio to 680 AM and you'll hear the traffic every 10 minutes. It will let you know where the problems are. Good luck.