any recommendation over gtx 1080 ti?

[helloeverybody]

I was initially going to just add a 1080 but it seems more cost effective to add a 1080 ti to my system. This is purely going to be used for mining since i dont game much these days but thats not to say i wont again soon. Can anyone recommend a better card or even a few cards that would make for a good mining rig. Ive currently got a 970 mining so it will just be used to add to that.

And on a side note im seeing a big variation in prices of 1080 ti from around £630 to about £800ish. I will probably over clock whatever i get anyway so is there any difference in these cards for mining use?

Thanks