I-5 isn't bad until you get to the Siskiyou pass between California and Oregon. Sometimes it's not bad and other times you will need chains to make it over. The best bet would be to keep an eye on the weather and the pass conditions. If you take 101 you wouldn't need to worry much about the weather but it's a slow drive with all the small towns you would have to drive through. You would still need to go over a pass in the coastal range to get back over to I-5. Coming up through Nevada and Bend is an option as well but once again you'll need to cross over a pass in the Cascade range. In short you're going to have to climb a pass as some point. If it were me I'd take I-5 and keep an eye on the weather and pass conditions and plan my trip accordingly.
As for weather in the Eugene / Springfield area in March. I'd plan on 50's and overcast with frequent or steady rain. It's been pretty dry so far this winter so you could get lucky.
Brad