If I'm developing a website, in terms of generating revenue, would it be better to immediately sell advertising once the website is finished? Or would it be smarter once the website is launched to build an app first, and then sell advertising?

If you are looking into monetizing your website then it strategically it makes no difference. The sooner you start selling advertising space the sooner you start earning. Most people sign up with Google AdSense to sell their advertising space. Adsense is like telling Google, "if you have people wanting to advertise online, you can let them advertise on my website" and Google says "ok, but I will charge a commission". What that means is most advertisers dont even know they are advertising on your website until their advert has already been shown on your website. It also implies that you start making money no matter how little your web traffic is. If you chose to sell advertising space yourself then you need to find advertisers online and also provide then access to all the analytics for example, number of clicks, number of impressions, number of conversions, time of clicks, location of clicks, demographics, and manage all the backend software which is going to cost you a bomb. The above suggestion is based on my limited understanding of your website. If you tell me more about it I might be able to give you more info on this. Good luck! Alok

Before having advertisments in your site you have to supply data to your clients on how many clicks, impressions, visits, etc you have received. It will give them a good reason if they should spend on you or not.