I am planning a trip to the Southwest in October, and due to time constraints, will only be able to see either Lake Powell or Bryce Canyon. Anyone have any opinion as to which is more worthwhile to see?

One vote for Bryce. (To us, Lake Powell didn't have "it"! The arch was beautiful, but I'm not sure it was worth the boat ride back and forth on the lake.)
Before you go to either place, however, check to make sure what services are available and what the weather conditions will be that time of the year. It can get cold there!

Lake Powell is perfect in October, in the 80's. I wouldn't go there just for one day though. Bryce is great for a day, better for two. It's at 8,000 ft., so it will be cold at night and cool, but pleasant during the day, great hiking weather. If you only have one day, make it a long one. Get up for the sunrise - it's worth it! Hike down in the canyon for a real appreciation of its beauty and uniqueness. Finish off the day with a visit to a couple of the further viewpoints.

Comparing a beautiful limestone outcropping to a lake is like comparing apples to lettuce. Which do you prefer?
My vote in a hearbeat goes to Bryce, the most charming (to me) of all the National Parks.

BRYCE!!!!!