Maybe an attorney or accountant can correct me, but I think that it doesn't matter which you do.
Running a second LLC will mean that you have to do all of the same reporting to the IRS and local government twice, have to keep two sets of books, etc.
Making it a division of your existing company means that you will have more complicated accounting if you want to track the profitability of both ventures.
In the end, it just matters what makes more sense to you. I don't think there are any legal requirements either way, but I want be sure to say I am neither and attorney nor a CPA. Just a guy who has started a few companies.