Apple and Google both give headline statistics of how well their respective app stores are doing, generally at their summer developer conferences. These are rounded numbers at scheduled events and they're not always comparable, but they do give us a sense of what's going on.
Last summer, at their developer events, both Apple and Google gave numbers for the money they had paid to developers in their respective app stores: $5bn in the previous 12 months for Google Play and $10bn for the iOS App Store. Given Android has double the user base of iOS, this meant that the average iOS user was worth around 4x the average Android user in app store revenue.
This year Apple gave the same number - $10bn (more precisely, it gave a cumulative figure of $30bn this WWDC versus $20bn last WWDC). The lack of growth may be partly due to rounding but still implies that people are spending less on average, since the user base is still growing. Google gave no number at Google IO but it gave one earlier in the year of $7bn. It looks as through Play is growing faster than iOS and might overtake it this year (unless Apple is rounding down very aggressively - certainly the uneven shape of the graph in 2013 is due to rounding).
Since Google Android has close to double the number of users, this implies that the average user is spending perhaps half as much as the average iOS user - a change from 1/4 a year ago but still a big gap.
Meanwhile, this was the first year since 2013 that we could compare downloads.
Google Play had 50bn app downloads in the last 12 months and iOS had 25bn, with Play appearing to be growing faster. Since, again, Play has more users, this implies roughly the same downloads per user on Android and iOS.
Incidentally, these numbers show annualized consumer spending on apps of around $25bn, and 75bn apps downloaded in the last 12 months.