What 4runnerguy said. The wife's family started making the pilgrimage from Dallas to Estes Park in 1917 via train. She grew up making the trip pre interstate when it was a hard two day trip via 287 then across New Mexico to Raton and up what would become I25.
We moved to Colorado in 1980 and used the traditional route several times a year to return to Dallas to see family and friends. When the population along the Front Range exploded in the 1990s, I25 between Longmont and Colorado Springs became increasingly congested. About 10-12 years ago, one of our Texas friends suggested taking 287 to I70. We found it to be much less congested and actually quicker. There are no passes or steep grades.
If you decide to take the traditional route via Texline and Raton, be sure to grab the Amarillo loop and take Ranch Road 1061 via Boys Town and 385 to Hartley. It is a beautiful drive and you miss the traffic in Dumas which can be heavy.