I would say route 1 or 2. SLC traffic isn't that bad if you hit it mid day. And to be honest, I have even been in it during rush hour and it wasn't bad either. With route 1, fair warning, when you see the signs saying "no gas for 100 miles" they are not kidding. Some people say it is a boring route, but I think it is good to do once just to appreciate the vast openness. But that is just my opinion and yes, I have done is several times. The other commenter was right though - if you do go this way, you could and most likely will, run into some seriously strong winds at some point.
#2 is a very pretty route as well, but you will have some huge passes, as mentioned.
Good luck. I drove from KS to Seattle a couple of times, so I know how much fun you are about to have!