Like Brian said it depends.
If you are going to keep it and like the older body style better, buy the '06.
If you are likely to not keep it, buy the '07. You can usually recoup the difference in cost when you resale. People perceive the older body style as old, even though it may only be a year or 2 old.
Now you have the other factors. Being a new model, are all of the bugs worked out vs. what can you get in the new one that isn't available in the old one (see I told you people think of them as old
).
I had a '97 K1500 that I really like and had put about $8k worth of options on. When the '99 Silverados came out, I had to have one because of the heated leather seats, heated mirrors, and the Auto Trac 4WD. I absolutely love my '99 Silverado, have had very few problems and those were minor. My truck was ordered the last day you could order '99s (should have waited for a 2000 and gotten the 4 door extend cab) but I know of folks that have early model '99s and have had a lot of ABS & Window problems. Looks like GM got their stuff together mid-model year.
Now I'm wanting a 3/4 Crew Cab w/ a long bed and Duramax/Allison combo. I don't want to buy now though because I want to wait until 2008 when the redesign comes out for the heavy duty trucks.