You could also go for the hybrid solution where you initialize the system with some advertisements but give your users the option to pay for a subscription without the ads. If you go for advertisements, you might want to start of with Google AdSense or something similar since they take care of alot of stuff for you and often provide you with the means to better target your ads.
You might then wish to create your own targeting system...
As for the demographic info attractive to advertisers, it depends. If you can convince them that you have an AI targeting system, it may help. But AdSense already seems to provide such a thing, but it is limited in power.
Like dj_segfault was saying, if you need to find out any info, like the optimal subsription price, try some out, log the results, and infer the optimal cost. You could even automate this process. Although it might be difficult to implement, it would optimize your decision process.
The beauty with a login website, is that you have all this data about your users. If you implement the right tools, you can optimize your advertisement management system, your subscription system, etc.
Yet all this is very dangerous for it may get out of hand if you are not careful; it is a double edged sword.
While revenue is important, think about your users before anything. If you can, create, establish and maintain the website for them, not for you or for your shareholders; exist for them more than for your own self.
Do not maintain a web site in order to make money; make money in order to maintain a website.
Hope this helps.