Normally, I'd say US20......easier, faster.  However, you have time.  Also.....going thru Redmond and using the cut-off to Sisters is a HUGE improvement over coming in thru Bend and finding your way to Sisters. I have no idea what the city planners were doing ......but the connection on Hwy 20 east of Bend and Hwy 22 west of Bend to Sisters is CRAZY.....  :)  And the other poster was right....the John Day Fossil Beds are cool.
Not sure how paulj expects you to get to Fort Rock from Hwy 20......but that wouldn't be my choice.  It's WAY out of the way.  If you come thru Bend, and haven't done it yet (or for that matter, while staying in Sisters, check out the High Desert Museum south of Bend and the Lava Caves).  :)