It is better to buy R9 390. You can get more hash per rig and so the overheads of the rig is reduced to achieve the same hash.
You mean R9 380(x)? I don't see any 390's for less than $300 probably because they're all 8GB, and 4GB 380's are $190 and up. (Not looking at used market of course...)
I think R9 390 is better for the long term. It has 8GB memory. The Ethereum will convert to PoS next year, but Expanse might not. So if you continue to mine, you need larger memory.