I am debating what to get. I have a custom loop and I am replacing 2x980ti's with ek blocks. I was leaning towards the EVGA Hydro Copper but I don't care for the looks as much as the clear EK blocks. I like the looks of the Poseidon and the idea that it can be air cooled if I ever needed to. I don't know if the performance will be as good as the Hyrdo or EK block though. I also like that the Poseidon has PWM fan headers that I can use. My mobo only has a single PWM header that I am using for the pump. My other fans I need to control using Speedfan which is not ideal. I know I can get a fan controller like Grid+ V2 but the Poseidon header could work well. 
 
They are all about the same price within a few bucks. If I do go with an EK block I will probably get an EVGA card that has a back plate that is compatible with the EK block. 
 
What do you guys recommend? 

  I hear you, lol.  Yeah, it's tough making, justifying, these decisions.  There's lots of room to second guess yourself.  A bit of a bummer that there's no 'super nerd' badge that one gets for angling towards high performance computing...I guess rocking games on ultra settings will have to do, lol.  Thanks for the 'helpful' designation.

I have had the Poseidon for a couple weeks now. Using one HDMI and two display ports is working perfectly with Nvidia surround. I assumed it would and it is. 
 
The issue I am having with the Poseidon is the external fan headers. They will not detect any fan that I have tried. I tried using a Swiftech PWM 8 fan splitter that is powered directly by the power supply. With the splitter in place the fans all run at 100% and that doesn't change while trying to calibrate the fans. With the fans I tried connected directly to the card they would power down and up while calibrating but it would always say detect fail. That was the case with both 4 pin PWM and 3 pin fans. I tried different drivers, different revisions of GPU Tweak II, etc. Nothing worked. I finally gave up and ended up getting a NZXT Grid+ V2 fan controller. That is working perfectly for my needs. 
 
One of the reasons I went with the Poseidon is the external fan headers. Well they aren't working as advertised. It does boost over 2000 mhz at stock settings even while only being air cooled. I spent a few minutes overclocking with the fans set to 100% and it could hit close to 2100 mhz but would crash over 2100mhz. Maybe once I connect it to my loop it will be stable above 2100mhz but that is unlikely and rare for a 1080ti. 
 
Should I return the card and get something different? Send it in for a RMA and cross my fingers I don't get a refurbished card back? Just ignore the fact the headers don't work as advertised and keep the card?