Upstream vs Downstream O2 sensor | Maintanance

I had my car checked up last year and the mechanic said that I will have to replace the oxgyen sensor. I at that time did not have it replaced. Recently I noticed that my car Mini Cooper 2011 started jerking while accelerating and also too many jerks during idling with the AC is on, it looks like that spark plugs have been plugged by the carbon built up or the spark coils are bad.

Assuming that the spark plugs have been clogged leave one conclusion that the O2 sensors are gone bad, now Mini cooper has upstream and downstream sensor, so I would like to know which one is more suceptible to going bad? Replacing both is gonna be a financial nightmare. I had replaced spark plugs just last year and now I will have to replace them again so as far as O2 is concerned I want know which one should I replace as I cant replace both.