O2 Sensor or Cat Plugged?

I have a high milage car (146K miles) with a P0430: "Catalyst System Efficiency Below Threshold (Bank 2)" MIL code. Does this code point to an O2 sensor problem or the cat itself? Neither is a cheap fix, but the sensor is obviously the easier of the two.

Am I on the right track or are there other possibilities besides what I've listed?Thanks for any help you can offer.