I would take I-15 directly to I-70 in Utah. I-70 through the San Rafael Swell in Utah and across the Colorado Rockies is not to be missed. The most scenic stretch of interstate I have ever seen.

Here is a photo taken on I-70 in Utah, at about mile 116 looking east through the Swell:

http://img37.exs.cx/img37/892/100_0852.jpg

And here is one taken from The Summit, outside of Frisco, Colorado along I-70:

http://img37.exs.cx/img37/3489/100_0902.jpg