The difference between the app stores Google Play and iTunes
iTunes is an app store made by Apple, it has everything that Google Play has, however, iTunes' main focus is music.
Google Play is a media market that unified all of the google services on the android under one name. It was originally just an add-on to the android market to help give a little more to the customers. But with all of the Google services on the anroid markert, they decided to do away with the android market and start up Google Play.
The difference is prices on both stores is the same, but developers prefer developing apps for iTunes instead of Google Play because they'll actually get paid. Android users have found ways to get almost anything in Google Play for free, therefore, developers like to stick with the market that they know they'll make money from.
What Google Play is today
Google Play today is simply all of the previous Google services that were on the android market combined into its own media store. Google Play's main focus is media in general, it wants to stay on top by providing the best variety of things to do. When I say that I mean it wants to have the best selling books available, it wants to have the hottest songs, it wants to have the newest movies and videos, and it wants to have the apps that catch people's attention the most.
Cost to submit an app
The cost to submit an app to iTunes is $99/year for as many apps as you can put out in that year. The cost of submitting an app to Google Play is $25 and it's a one-time fee and you can make as many apps as you want. One possible reason for the huge difference in price could be that Android users have found ways to get a lot of apps for free.
Honestly, which app store is better is up to you, and is a complete matter of opinion and yours may be different from someone else's based on what you like more. Someone that likes more music on their phone than anything else would probably be more into iTunes than Google Play. Someone who likes likes a lot of different apps would probably like either one because they both have the majority of the same apps. A reader would most likely want to use Google Play because it tends to have more selection to choose from with books.