the best route on my map in the field guide is the lower route
it is a bit adventurous. Many people might think too much. On the other hand, I'm pretty conservative these days.
Trip recommendations, current conditions, and other trail related Q&A
Sounds like you already have received a lot of good advice, but here is a link to a report from a trip around South Sister (starting and ending at the Pole Creek TH) a friend and I did five years ago. I remember the trail between Middle and South Sister fondly, and we never had any trouble following the trail.