I'd say use the "scam" domain since you already have it... but create a different template feel and look so users will not know it directly related to your product but it talks about it....

And then write something along the lines of: Is Product X Really A Scam? and then continue by listing attributes that explain otherwise, followed by a direct link to your site...

You'll get tons of traffic from that link and the domain will be put to good use..