Fat transfer to buttocks or hip, which one is most dangerous? Which one has more of a pulmonary embolism risk?

This is an excellent question given all of the recent media coverage of BBL mortailities. In fact, the plastic surgery community was first alerted to the risk of Pulmonary Fat Embolism in a paper published in in 2015.
I have been performing this procedure safely for 15 years and the new data emerging about the increased mortality rates is disturbing to say the least. ALL fat transfers carry the risk of PFE.
A pulmonary fat embolism (PFE) is a mechanical blockage of the pulmonary veins by fat particles. It occurs when an injection cannula injures a vein, allowing injected adipose fat to enter the vein and block it. Fat particles can also travel through the pulmonary veins to the heart. While not all PFE’s are fatal, many do result in death within the first 24 hours following surgery.
The injection technique is what's important here - not whether you are enhancing the hips or the buttocks, but whether or not the surgical plan and actual surgical techniques minimize the risk.
If you are contemplating a Brazilian Butt lift, you need to be aware of the risks, including PFE, and minimize risk by choosing a board certified plastic surgeon, having your surgery in an accredited surgery center using IV Sedation with Tumescent Solution instead of General Anesthesia. You should also ask your plastic surgeon for details about the actual injection techniques that will be used during surgery.
See the web reference for more information about my credentials to speak on this issue and for more detailed explanations of how you can minimize risk. Asking all of the right questions will help ensure you're choosing the right surgeon to trust your life with. 

Patricia,Fat transfer to either the hips or the buttocks is very safe.  There is probably a slightly higher potential for embolus with fat transfer to the buttocks due to the increased vascular supply.Good luck with your decision.Dr T

I think that safety is paramount whenever a surgeon treats a patient. Both injecting to the hips and buttocks are safe.  Because the buttocks are more vascular, there is a higher likelihood that fat could get into the blood system via that route, but it is extremely rare.