Quixit :
r9 290x is faster, and it's slightly faster than a RX 480 if you're comparing to current cards. For reference the Geforce GTX 1070 is much faster.
Do you have benchmarks for that? My friend is in the market for a GPU, and I found her a used R9 290X for $185 + shipping. She also has the option to wait until the RX 480 comes back in stock somewhere, but I wasn't sure which is technically a faster card.