Running on Sidewalk or Street

I've run for a number of years and have regularly heard that, whenever 
possible, one should always run on streets instead of sidewalks because 
the asphalt of a street is "much softer" than the concrete of a sidewalk 
and it's better for your joints.

I've never found this to be true in my own experience -- streets have 
never seemed any softer than sidewalks.  

I assume part of this arises because the asphalt on a pedestrian/bike 
trail   is softer than the asphalt used for a city street.

Is there any significant difference between a street and a sidewalk, 
especially if one has a decent pair of running shoes?  

Googling around, I find a lot of anectodal evidence but very, very little 
hard evidence.  There are a number of cites of a "Runners World" article 
that rates surfaces with numbers 

http://www.runnersworld.co.uk/news/article.asp?UAN=152

but the numbers appear to be purely subjective judgements.

This article cites both the Runners World piece and four studies in 
medical lit, which seem to cast a lot more doubt on any difference 
between concrete and asphalt:

http://www.signonsandiego.com/uniontrib/20060118/news_1c18sciqa.html

Does anyone have a good cite for how much difference there is between 
concrete and asphalt (as used for a typical sidewalk and street)?  
Ideally, this would put it into perspective for running on, say, a dirt 
trail, a rubberized track, and a path made of the purest, ****niest  
diamond.