both of them actually. It just depends on your accent and the specific way you read it. personally i pronounce it He kayte when i'm talking about the great goddess but during ritual or chanting i call her hek uh taye, so it all depends on who you are and what you are doing, and how you read it. Don't worry so much about prounoucing it right till after you get a little deeper into it all.
There's a number of different modern pronounciations, all of which are identifiable (i. e. people will understand who you are talking about.) But going back to the Greek (which itself has different folks arguing over pronounciation, depending on the dialect, and the spelling and pronounciation of the words themselves changes according to their placement/purpose in a sentence)....
' - like the e in etch, but with a soft breath h sound, not a hard h
- hard c, like in cat
- soft a, like ah
- like in tardy
- like the e in error or the a in ace
Greek vowels have a 'quality' of being short or long, which refers to (in Classical pronounciation) how long the vowel is held for. Epsilon is always short, alpha can be either short or long.
In Greek words, the accent falls on one of the last three syllables; in the Greek spelling of Hekate, the accent mark (which I can't do on this computer) falls on the central alpha.
So, roughly, Hekate is heah-KAH-tae. Mostly.
Other good links on Hekate:
Oh--and if anyone is saying she is merely a Goddess of the night, witchcraft and Underworld, and is a Crone, they are dead wrong.