Yes, until a year or two ago AMD was better for BOINC, their design more suited for heavy computational workloads. However, I would say that Nvidia has overtaken AMD recently, especially with their GTX1080 series which have unparalleled FP32 performance. Not to mention that there are still BOINC projects which support only CUDA and therefore only Nvidia (like Asteroids@home or GPUGRID). Looking forward to Vega and better performance from AMD.