The problem with ads is you need eyes on those ads, and the ads invariably detract from gameplay, and even performance.
Then a lot of people say iap is the way to go, but it's an even worse sh*t show to get working.
What I will probably do in the future is release to the portals with adequate protection, and if it does well there consider doing a free version on mobile.
If it does well there, try to do either value added iap, or ads for bonuses.