Like many words in modern English, these two variations are both correct. That said, “preventive” is more common and there are slight variations in usage. According to Merriam Webster, “preventive” can only be used as a adjective, while “preventative” can be used as a noun or an adjective. Let me give you some examples:
From the Chicago Tribune: Preventive health care allows your doctor to find potential health problems, before you feel sick. (adjective)
From the New York Times: But we cannot allow ourselves to forget for even a moment that force is effective only as a preventative—to prevent the destruction and conquest of Israel, to protect our lives and freedom. (noun)
Now let’s be honest: How many times are you going to use “preventative” as a noun? Probably not very often. So, if you have no personal preference between the two spellings, I suggest sticking with “preventive.” Of course, if you’re the nonconformist type, “preventative” is still a word, so you might as well use it. Who knows? Maybe it will make a comeback.