I understand, but I spoke with my dyno guy at length on the topic. He's been tuning cars since I got into Mustangs in 1998, and he said to ONLY buy the OEM Ford part. While trying to diagnose tuning issues with cars on the dyno, he's hooked his scope up to aftermarket O2 sensors and found that even Bosch sensors were producing a different wave patter than Ford OEM. This was wreaking all kinds of havoc and making the car difficult to tune. He swapped out the aftermarket ones for the Ford OEM sensors, and voila... problem solved.
For me, the extra $20 each for Ford sensors is irrelevant to me cost-wise, so I just buy them.
Paul.