You've gotten great comments so far, and I totally agree that you should never backtrack if you can avoid it. New roads are the best roads!

If you want to get a taste of old Route 66 and the fun along that route, you could also leave San Francisco by going south on I-5 to Lost Hills; east on 46 to Famoso, south on 99 to just before Bakersfield; east on 58 to Barstow, then east pm I-40 to Albuquerque; north on 25 through Sante Fe and onto Denver. This does add a couple of hundred miles to your trip (about 1550 miles and 25 hours driving) but it's very different scenery than what you'll be seeing on your way to San Francisco and would be worth doing, in my opinion.

You've got lots of options. No matter which route you pick, you'll see some amazing sights!