Interstate highways are generally a better option for ANY place you want to go if speed is important to you. If you want to actually SEE anything during the trip back roads are your best option. Main difference between 80 and 70 is the number of trucks you like traveling with. 80 is the major east west truck route from coast to coast (not that they are scarce on 70, just less of them.
At ANY given time during warmer weather, either of those routes will be under construction an many places and in major cities that construction can be pretty much a constant thing but even then they are usually the most economic (in time and money) way to go.