I’d bet we all use one of these cemetery-searching websites. Maybe some of us use both. Is one better than the other? What’s the difference between the two? Well!
The focus of Findagrave.com is on the individual. Contributors can create a “memorial” (web page) for anyone. There is no requirement for a photograph of a headstone or anything else but a name. BUT a Findagrave memorial can potentially contain much additional information such as biographical information, photos, obituaries and information on other family members.
The focus of Billiongraves.com is photographs of headstones with their geolocation tags. To submit a photo to BillionGraves, one must use their app and a GPS enabled camera or smart phone. Having these GPS coordinates, can save hours of wandering time when visiting a large cemetery. 
Both websites are free to use, so you decide. Just do realize that all of the information on these two websites is secondary, not primary (with the exception of the location of the grave). Despite efforts to ensure accuracy, there is no way on either site to document the primary sources of the information…. with the exception again of folks adding other forms of documentation. 

Even though a date is “carved in stone” the stone is a secondary source, not a birth or death certificate.