Help deciding between GPU's

[Scarab575]

So I just need a bit of help. Essentially, I can get a couple of different cards. It's between 3 different versions of the RX580 or  RX570. The costs are for 6 GPUs. Either $1536, $1377, $1108 for the 580s or $1062 for the 570. I'm worried mainly about getting the best memory on it, like samsung or hynix from whatnot have seen. But also the differences between the actual cards themselves.

Does the core clock really matter? I understand undervolting and overclocking.

Thanks in advance!!