The east route is supposed to be faster, lower altitude, better roads. The west has better scenery (depending who you talk to).
If you stay on the east side, you drive through the middle of Colorado — Denver, Colorado Springs, and so on. And there’s a lot of stuff in Colorado Springs — Pikes Peak, Air Force Academy, a few gold mines, Focus on the Family, and Garden of the Gods. Garden of the Gods is a set of striking geological formations. The park’s admission is free. What I find remarkable about so many of the Colorado Springs landmark — they are located just a few blocks from downtown. Weird.