Which is better to learn first, iOS or Android app development?
As a developer with the prerequisite skills but not the actual experience building dozens of apps for both operating systems, it is unlikely that you would finish an app of any substance for both iOS and Android in 3 months.
Which to start with? Android is often considered the easier way to get started with native mobile because Java is simply more common. We also see more developers who do not have hard CS degrees start with Android. Conversely, iOS needs are the more common dev languages encountered by a mobile app development agency such as ours. The reason is that iOS is the reference platform for most mobile product. And to some extent, in-house teams feel that ‘we got this' even when they don't....Exceptions include apps that require OS customization, rooting, or permissions that iOS/Apple do not offer; these lean toward going with Android first.
We note that clients without strong in-house mobile development often have someone working on the Android version - presumably due to Java being a more accessible and common skill set.
We find that progress with iOS is related to educational background and experience with object oriented or low level languages. This is often not what a learning developer is happy to hear. A professional with a good CS or EE degree can get underway very quickly with iOS. A person with an IS degree or business degree may get started and do a few apps, but may not end up as an iOS developer for the long term.
There are cross-platform tools such as Titanium, Sencha etc. We eschew them for straight up native development because we can, and it's is not cheaper to use cross platform tools; it's merely more accessible to more people. Exceptions may include simple apps or enterprise apps without concern for UX/UI optimization.
In summary, I would not bet on being done with both platforms in 3 months unless you know yourself to be particularly gifted as it pertains to new dev environments and have excellent prerequisites behind you. If you proceed without the expectation of that deadline, you may find that Android is easier to start with.