Which road shall I take from Denver, CO to Salt Lake City, UT: I-80 or I-70 w/ US-6?

Based on your criteria, the I-70 to US-6 route appears to be superior.

• This route has T-Mobile 3G coverage along most of its route, whereas I-80 has little or no data coverage west of Cheyenne.
• This route is slightly shorter, though the difference in distance is trivial. As both routes travel through mountain passes, fuel usage should be similar between them.
• Aside from far eastern Utah and far western Colorado, this route has many possibilities for scenic overlooks, stopping points, etc. The Wyoming route has a similar stretch of nothing, so it's even either way; neither route has a good "halfway" point that's approximately half way. Thus I'd recommend starting in the morning and making a one-day drive of it, by either route.
• If you start your westbound trip in the morning, the afternoon sun won't be in your eyes as you travel north on US 6 into Salt Lake. Nor, when you leave Salt Lake in the morning, will the morning sun be in your eyes as you travel south on US 6.
• US 6 is divided highway (dual carriageway) much of the way (about 2/3 of the distance) from I-70 to Salt Lake City. The state highways won't be.

Here's some things to consider.

1. Some people go I-80 to Laramie and down through Fort Collins into Denver. However, Fort Collins has developed and more stoplights will slow you down. Now some people elect to go to Cheyenne and then down I-25 but this adds some distance.

2. Where you want to go/or traveling from in the Denver area is a big factor in your decision. If you want to say go to Lakewood or Littleton, it might be faster to go I-70 from anywhere in Salt Lake. But if you are going to say Thornton or Broomfield or Aurora, it might be slightly faster to go I-80/I-25.

3. Most people prefer the scenery of I-70. It is a bit tougher on the car but the scenery is more varied and the elevation changes more extreme. Both routes can be brutal in winter with different challenges. However, I-70 from Denver to Vail can be especially difficult on Sunday evenings or Friday afternoons depending if you are going toward/away Denver, where the I-25/I-80 routes are more consistent with traffic.

4. On I-70 there will be more tourist opportunities from the Grand Mesa to Vail to Silverthorn, and on the Utah side you can even slip down into Moab and visit Arches.

5. Take into account construction. Both I-80 and I-70 have seen plenty of it but the I-70 construction seems to be a bit more complicated with more delays. I've sat in my car for nearly an hour getting through Glenwood Canyon, another great scenic destination by the way.

6. From Green River to say the Wasatch front, you will spend time on U.S. Highway 6 a divided Highway which is a dangerous highway statistically. Usually you can travel by decent rate of speed but it is a two-way highway nonetheless. Same goes from Laramie down to Fort Collins cut off but the distance isn't as great and as I said some people stay completely on the freeway by going all the way to Cheyenne and than taking I-25.

Nobody is talking about traffic. The traffic on I 25 between Cheyenne and devore is absolutely horrendous. Either direction. Especially during rush hour. I am a truck driver and I avoid it at all costs, if at all possible. Going I 70 will give you some of the best scenery this country has to offer. Take a break at the rest area at the top of Vail pass. You won't be disappointed. All that being said. I 70 between Denver and the Eisenhower tunnel can be just as horrendous as I 25, as far as traffic goes. If I didn't have to drive a big truck I would try us 40. Again some of the greatest scenery in the country. And I would think far less traffic than either of the other 2 choices.