The game will be launched on iOS because this platform has the most paying players on average. The downsides are that the revenue share concentrates in fewer games and that the time to get it approved for the market is considerably longer.
Have in mind that developing just for one OS might limit the exposure of your game on the different app markets.
Google Play and the App Store are the most popular markets for mobile gaming which is relevant for the multiplayer feature. Although, Amazon Appstore is the market with the best sales margins.
Windows Phone is growing fast on some markets and there is much less competition. Maybe it’s a good option.