Best in-game ad delivery system?

Hi, I'm developing a game in Unity (I do graphics and hired a coder), and want to make it a free game with in-game advertisements. I realize there are several ad delivery systems out there (Unity Ads, AdMob, Flurry, iAds, etc), and as a novice I'm really not sure where to start. I'm looking for feedback from experienced developers.

For example:
Which one is easiest to integrate & customize?
Which pays the best and yields the best results? (or are most comparable?)
Can we customize with relevant ads to the target audience, or is it entirely up to the ad system's algorithm?
Anything else to consider?

My game will be for iOS and Android, and I would like the eCPM model where users don't have to click on the ad in order for the developer to get paid. I will have to get an estimate from my developer how much it will cost additionally to implement advertisement functionality into the game, so ideally I am looking for something that is on the quicker/easier side to integrate.

And I realize this is a very wide-open and subjective question, but how much on average might I expect in terms of time and cost for a seasoned developer to incorporate ad functionality into a game?

Thanks for any insight!

 

I found both UnityAds and AdMib trivially easy to implement.

You can also use a mediation service that hooks up to several ad providers.