I would recommend the OR62 route. It's the most direct. It gets higher than the other choices, but none of the grades are difficult. The grade on 138 to 97 is just long and straight. There are some curves as it follows the Rouge River, but nothing exceptional. There may be some logging traffic, but most of the traffic will be rvs taking in the sights on the way to/from Crater Lake.
I5 north or south will have more truck traffic. US97 is also a major truck route. The one time I took I5 over the pass (in a car) it felt like I was playing hopscotch with the trucks. It might have been different if I was in an RV and stuck to the slow lane (but trucks tend to speed up on the downgrades, even in the slow lane).
138 from Roseburn is also pretty, and not too steep or curvy, but a little more out your way. OR58 is one of the easier Cascade routes. The climb from US97 to the pass is barely discernible. But it is also busier.
Diamond Lake and Lemolo Lake at the top of 138 is a nice area to stop and camp, with lakes and waterfalls.
