You should reall over look the difference in clock speed here.
Comparing the company Zotac to EVGA is like comparing a generic Dynex TV to a Samsung TV in a way. EVGA would be Samsung.
Out of all the hardware companies I can think of, none of them have better technical support than EVGA.
EVGA had 2,3, and lifetime warranties on their graphics cards. Any graphics card ending with the model number ending with AR or A1 carries a lifetime warranty, KR is 3 years, and TR is 2 years. You do have to register the card within 30 days of purchase.
EVGA has a step up program that lasts 90 from your original purchase. If you bought the EVGA GTX 560 and registered it right away, but decided you want the GTX 560ti or GTX 570 then you can trade your card in. The will give you full value for your card minus any rebates.
For any set-up or warranty work you will have to upload your reciept or invoice onto their website.
I can't comment on Zotac's customer service.
Go look up the reviews at newegg. Some of the EVGA cards have hundereds of reviews while a Zotac card has about 10 reviews.