There is no "right" way to spell it, because Nicole is not a Japanese name.
Both spellings are possibilities, and it's your choice.
I also agree using the ー is my preference. COLE is the stressed syllable in English. Japanese doesn't stress syllables, so making the O twice as long with that symbol is the closet you can get to stressing the O. It sounds the closest to the original English as possible.
But you can do the other if you wanted to. Each syllable will be pronounced for the same length of time.
note: the Japanese version is --- nee ko roo; where KO is pronounced twice as long if you include the lengthening mark, and the R is a Japanese R (between an R & D), and the OO is Japanese version, like OO in English food, but with the lips slighlty less rounded.