Mainstream vs. Sidestream
Evaluation of a compact device for capnometry of main-stream type compared with one of side-stream type in a postoperative care unit
An article from the Japanese journal Masui, concludes that mainstream monitoring is more accurate than sidestream in monitoring breathing in the recovery room. Sidesstream gave 5 false apnea readings.
I haven't been able to get the full study, but from my experience, can tell you I have had the sidestream apnea alarm go off at times even when the patient was breathing. While I believe the nasal cannula capnography is very important, it is not completely reliable due to positioning and patient anatomy.