Of the two, I-70 would be more scenic but quite a bit longer in miles. If you have two days to do it, I would suggest a more scenic, but slower, route. Take I70 into the mountains to US-40 at Empire then follow US-40 west all the way to Park City. The last few miles will be interstate.

My mapping software shows that route to be 464 miles and taking 7:30 enroute without stops. Of course, with all the gorgeous mountain passes and parks, you'll want to stop so I recommend using two days with an overnight in Craig, CO or Vernal, UT.

There's nothing particularly noteworthy about either place - they're just convenient stopping points.

US 40 is the shortest and fastest way to Park City and reasonably scenic. Equally fast is I-70 to Rifle and the local roads to Vernal and then US 40 to Park City. The route across NW Colorado is very rural and you will be luck to see five cars.

I-70 is indeed very scenic and should be seen at least a few times to really appreciate it. If you cut north just past Green River on hwy 6 it is about 9 hrs.

Maybe a little bit faster would be to cut north at Rifle up to Meeker and west to Vernal. We just went this route for the first time and enjoyed the less traveled way, though I expect to continue to use hwy 6 and I-70 when we drive it.

If you are in a hurry and are not bothered by scenery, if you hop on I-80 and cut through southern Wyoming, it is about an 8 hour drive from Denver to Park City. Used to do this trip annually, I have driven all of the routes recommended, above.