Are there any time pressures?
Tony,
The routes you have specified allow (& require) a mosey 'tude when traveling. Because they pass thru small towns, (with rigidily enforced speeding laws), you have to allow an average speed ~50 mph. There are places, of course, where you will reach normal highway crusing speeds (70+) but there are significant sections where you will be going 30 mph.
A professional team just did the entire Route 66 trip from Santa Monica to Chicago and back. Their site is excellent for choosing what you want to see. For more information check out Route 66 pages: http://www.roadtripamerica.com/links/route66.htm (look at the third link in the list).
The fastest route would be I-70 most of the way and then follow US-50 thru Nevada (this stretch is nicknamed "The Loniliest Highway in America") and then past Tahoe, over the Sierras and into Sacramento.
Mark