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.
in 1 year you can still go with 4 gb, so this is a very long term thing, and i'm not sure if it is right to remain with the same gpu for a long term when for the next year new gpu will be available
The R9 390 is an old GPU, but it is still faster than R9 Fury or Fury XT in the Ethereum mining.