I believe a large majority of the time it's the upstream sensor that goes bad, and replacing it takes care of the problem. Are you getting codes or other performance data that showing problems with both sensors and the cat as well?
AFA buying goes, I'm not aware of a specific O2 sensor which doesn't work ok on our Aveos - maybe someone else here has had problems and will post about that. Assuming none are bad, then just go for the best price. There can be quite a large difference on the various sites and stores. Earlier this week I paid $47 shipped for an O2 sensor for our Dodge Caravan from a popular online store. This sensor had a range of prices elsewhere, all the way up to $74 (same make/part #).