Comparisons -> Adwords vs. Direct Mail

Google Adwords	Direct Mail
Your ad goes in front of someone who is interested in your service.	Your ad goes in front of someone from a list. Perhaps they are interested in your service, most likely not.
Your ad goes in front of them right when they are looking for your service.	You ad goes in front of them at a random time.
Don't like the ad you're running? Change it!	Once you've printed them, they are printed.

The one close comparison between these two medias is in "cost per piece". For most big city Adwords accounts, the cost of a click will be about equal to the cost of a piece of direct mail. That said, I'd much rather have my message go in front of someone at the right time - when they are in the hunt for my services. It just gives a better return-on-investment.