I spose it really depends how far south in colorado you live. If I were driving from anywhere south of Colorado springs id cut down and take 87 to hook up to I40 at or around amarillo.
I-40 has its messes, but overall its a decent enough road to take across country. Arkansas and Oklahoma used to be really bad but over the past few years they have repaved quite a bit of it. Its not as slappity slap slap bumpy as it used to be lol. I've taken I40 across the country many times and every time it seems to get a little bit better
Cant say much about I70. only been on it once