A recent RCT suggests that, post-extubation, HFNC can decrease the rate of reintubations significantly even in healthy patients, when compared to regular nasal cannula. The advantage of HFNC over BiPAP is higher comfort levels (allows eating, speaking, coughing, no mask injury), better compliance, better secretion evacuation (it also humidifies them), while possibly decreasing dead space.
AFAIK, the same can't be said pre-intubation, although it can buy enough time in selected cases. Its PEEP level is in the 5-6 com H2O magnitude, even at high flows, so BiPAP is better for hypercarbia or CHF patients, or whenever you need more pressure support. Generally I use NIV in either form as long as I (fore)see significant improvement in the short-term. It can work beautifully in selected patients who would have increased complications if intubated (immunodeficiency, chronic severe lung disease etc.), provided that the factors leading to hypoxia/hypercarbia are aggressively treated. If it doesn't work in 24-48 hours, I intubate.
I never pay attention to the BiPAP flow rate, only to IPAP/EPAP.