AppFollow is well suited if you only operate in a few app store countries and don't have a high volume of keywords that you are looking to rank for. This is, of course, the free plan. Besides that AppFollow is well suited for communication with other apps - they have an open API and other connections to a ton of third-party analytical tools.
I always make sure that we are actively using Bing and Google. There is a tradeoff in your time when you use more tools, but there's also benefit in multiple perspectives. And don't forget that Bing is a very viable search engine that receives 500 million search queries each month. You may find that your business benefits from a relatively small amount of attention paid to Bing because search ranking there is easier (not easy, easier).
The user-interface isn't as friendly or appealing as Google's - very bland overall.
The way they present their Page Traffic report is a big turn-off. It's basically just a grid of numbers and arrows which is hard to interpret at a high level.
Very usable overall, quick to find information and conduct analysis for example of how a price of content is performing on Bing search. User interface is a bit clunky at times but easy enough once you get used to it.
While App Annie is extremely robust and pulls in data from acquisition channels, app stores, and elsewhere, it can be quite overwhelming at times. If you are looking for more simple and just keyword positioning then definitely look at AppFollow. Apart from the two platforms themselves, I've had numerous contact experiences with both AppFollow and App Annie. The winner is definitely AppFollow with a less aggressive approach and a higher willingness to go above and beyond to make sure all questions are answered. With App Annie I was bombarded with a ton of sleazy, sales emails to purchase their product.
I think that Bing does a good job of being a competitive product in a space that has very basic and insanely complex products. While Bing isn't pretty, it gets the job done and also gets you the right tools in a simple and straight forward backend UI. The ability to place Bing ads, track SEO and see how your site is performing, all from one backend interface is awesome!