Both the Aufderheide and McKenzie Pass have fairly substantial climbs. Both are worth the effort.
If I was to have to choose between the two, I'd choose the Aufderheide. Tough call though - the view of the lava fields and snow covered mountains from
Dee Wright Observatory on McKenzie Pass is a must see. The town of Sisters East of the pass is very cute also.
Just east of McKenzie Bridge, Belknap hot springs has showers. Their tent sites are way too expensive for me, but you could pay $7 for a soak in the pools and a hot shower. There are other non-developed hot springs in the area - Shifty mentioned Terwilliger on FR 19. Deer Creek HS is off of Hwy 126 and McCredie HS is off of Hwy 58.
If you're really short on time, there's also bus that runs to Eugene on the other end of the Aufderheide as far McKenzie Bridge.