Forums are easy to setup but hard to build good discussion on. As far as easy setup, ning is the way to go. I normally suggest getting your ning network to at least 500 members before investing the money in advertising. But I've had a webhosted forum and ning forums and I prefer ning.
Hmm...Great question. I have a few questions for you.
Has Ning helped you out in anyway such as connecting with others, promoting your business, and/or sharing information? Who is your target market? I am not familiar with hostgator, but I would like to suggest that you make a list of the pros and cons and look at what is already working for you and want you want to accomplish.
I let a comment under your profile with 3 possible options. Honestly, you wan to consider how many hours you can devote to administration such as obtain and confirming advertisements, billing and welcoming new members.