We travel that area going north and south a couple of times a year when going from ABQ to either Boise or over to Oregon. We usually hit the SLC valley around mid day and have found that traffic moves well, you can easily average 60 mph from Ogden down to Provo. Try to stay in one of the center lanes, the right lanes can be nerve racking with people diving in front to exit and other people slowly entering can make it a stop & go nightmare. We just completed the trip in both directions in June, there is road construction on I-15 (it's never ending) between Ogden and SLC and the road is bit rough, but nothing bad. I have never used I-215 so have no input on it.
We've traveled the route on almost every day of the week and I don't think there is a "better" day, Sat & Sun can be as busy as any week day.
BTW, for some unexplained reason my MH always gets at least 1 mpg better fuel economy when driving through there - could be all of the concrete roadway.
2018 Tiffin Allegro Red 33AA
2006 Jeep Liberty