i assume you mean what specs to consider if all the cards are the same gpu. like a bunch of 1070 cards, what do i consider?
the outputs it has should be as close to what you need as possible.
after that you are looking at the cooling it uses (1/2/3 fans, water cooling, hybrid), power needs vs your power supply's ability, and finally look into the board itself with things like power phases and supporting software you might need/want
many card models are simply reference design boards with custom cooling while some are custom boards as well with lots of extras vs the standard design provided by amd/nvidia
Maximum Digital Resolution
Standard Display Connectors
DP 1.43, HDMI 2.0b, Dual Link-DVI
Standard Graphics Card Dimensions
Thermal and Power Specs
Maximum GPU Temperature (in C)
Maximum Graphics Card Power (W)
Minimum System Power Requirement (W)
Supplementary Power Connectors
Which of those specs are useful to compare between GPUs? (example: CUDA cores, Graphics Clock, Memory Interface Width, ...) Which "numbers" mean better or worse performance when comparing between GPUs?
you unfortunately can't judge various gpu chips just by things like cuda cores and steam processors. like cpu's, clock speed and number of cores does not tell the whole story. that's why a 5 ghz, 8 core fx chip could not get even close to a 4 core i5 running at 3.5 ghz
every gtx 1060 will have those same specs because they are based on the same gpu chip. each brand will attempt to make their card different and "better" by changing the outputs, cooling system and usually clock speed out of the box. but in the end, other than temps and noise, there is pretty much no real world fps difference between them. this is due to the boost technology in the card.
boost takes EVERY chip and automatically overclocks it as high as possible until it hits power and temp limits. so a base reference card 1060 from nvidia usually gets within a couple fps of the top end 1060 super card that costs hundreds more.
now to compare between different gpu's like a 1060 vs a 1080, the specs mean little as well. they are usually different versions of the same chip architecture and thus perform differently.
in the end, when looking to compare cards whether the same gpu or different ones, you have to look at reviews. real world benchmarks is what tells the story of what a card can really do.
really about the only thing important the specs tells you is how much and how fast the VRAM is and how much power the reference design is designed to use. other than that, the rest is not very informative.