Can't answer your question directly without looking up the speed specs online, which you could just as easily do yourself. Bear in mind that the amount of VRAM is important for Resolve. Get 8GB if you can.

If you're buying from new then be aware that the RX 480 is out now, and more bang for the buck than the older cards you mention. Apparently it's a great performer with Resolve.

Personally I am waiting to see what V14 brings in terms of GPU acceleration before I buy a new card, just in case the Nvidia GTX 1080 performs better than the AMD cards. Unlikely, but you never know. By then I suppose an RX 490/495 might be out, or close, and likely a great card to use with both Vegas and Resolve.

If you're buying a used card then an R9 390X is probably a great choice and there should be some on the market as people upgrade to the later cards.

That's my thoughts based on very little research. If I'm wrong then someone please tell me.