What are the differences between the largest mobile ad publishers, and how do you evaluate them?

This is a cross-post of my answer to a question on Quora:

How does Millennial Media compare to its competitors?

My answer:

Here are some difference, as well as pros and cons between Millennial, AdMob, and iAd:

Millennial Media
+Success Stories: Millennial Media’s customers include 23 of the top 25 Ad Age brands.  Work(s/ed) with Zynga and Rovio
+Size: Millennial Media owns 17.7 percent of the mobile display ad market
+Ability to target via gender, age, household income, behavioral attributes, types of content preferred, device, carrier, and location, etc.
+Reengagement: similar to Flurry’s AppCircle, their App Engagement Program can drive users back to specific destinations inside your app, and retarget them with relevant messaging.  This is great if you can target and reengage those lost users that had a high LTV
+Full service or self-serve (very simple onboarding for self-serve)
+Free house ad campaigns to cross-promote products within your portfolio

AdMob
+Success Stories: Rovio used AdMob to expand their Angry Birds game to the Android platform
+Size: AdMob owns 24.8% percent of the mobile display ad market
+Ability to target demographics by gender and age (useful, since females monetize higher than males on freemium games)
+Offers in-app advertising
+Primary market is US and it also offers other regions to scale into when the advertiser is ready
+CPC is the predominant ad model, which is in-line with the kind of performance marketing that startups would best benefit from (especially when doing a burst campaign).  (CPI would be better.)
+Free house ads—for cross-promoting games across your portfolio
+Ability to target users on specific app platforms (iPhone+iPad, iPhone, iPad, Android, Windows Phone 7) and OS versions
+Easy, intuitive onboarding process in 4 steps
– No targeting of specific app types (for example, gaming apps).  This is a real problem because your likelihood of conversion will be higher if you can target this way.

iAd
+Size:  iAd owns an 18 percent share of the mobile display ad market
– Minimum cost of an advertising contract is $100,000
-CPM advertising model (was CPC + CPM)
+Sophisticated targeting based on factors including demographics, app preferences, entertainment preferences, location, device
-Ads can prompt users to do many things within the app without leaving what they were doing: download an app, send emails to their friends, share content, etc.
-Not self-service

The process I would use to evaluate spend for mobile ad publishers would involve asking the following questions (below) and seeing which would fit your products’ needs for user acquisition:

1)     What kind of ad model am I interested in? (CPM, CPC, Search CPC, CPA/CPI).  I would lean towards CPC, CPA/CPI
3)     In which platforms do I want to publish (desktop/mobile internet/mobile apps) Mobile internet/ mobile apps, with apps are preferable
4)     How much user targeting do I want? (country, carrier, time-of-day, user demographic, location device, mobile OS, behavioral) In most cases, Behavioral, country (those that monetize well), and mobile OS targeting would be most helpful, followed by gender and age
5)     What level of performance does this network offer? (average CTR ranges, conversion rate ranges)  The higher, the better – though isn’t commonly published information.  A test ad may be required to truly know what conversion rate your product should expect

(Jan 2012 market share, courtesy of eMarketer)

Hope this helps!