If you take I-70, you had better check with CO DOT first because Glenwood Canyon has been closed due to a large rockslide earlier this year. I rode I-70 east last year from Utah to Denver. It is an amazing ride.
If that's the case, then 40 is definitely the best bet...but, I wouldn't pass the opportunity to hit that canyon and scrape the floorboards a few times. Last summer I did catch one of those heavier-than-usual mountain showers going through there...created a little puckering effect....