As Bletchleyite says it basically doesn't matter. It's really a matter of personal preference. The difference is quite hard to describe - I suppose the best way of putting it is that iOS is a little more prescriptive in how you use it, which is either helpful or annoying depending on your disposition. There's also a lot of variation in different manufacturers' versions of Android, though you'll have no difficulty moving between them as they all have the same basic features and all run apps from the Google Play Store.
I use Android mostly, and if you are unsure, it probably doesn't make sense to buy an iPhone as your first smartphone unless you have a specific reason to - simply because they are comparatively quite expensive, and you may decide you don't like having a smartphone - but there's nothing fundamentally wrong with them.
Here are some more specific thoughts:
Pros for Android:
- If you buy e-tickets they are emailed to you as PDFs, and I find it's easier to organise these well on Android as the file management is less locked down.
- Transferring files back and forth between the phone and a computer is straightforward as you can just plug into the computer with a standard cable. This comes in handy if you want to take photos while trainspotting and then view them later on a laptop, for example, but is probably irrelevant otherwise.
- The phones themselves are generally cheaper for what you get and there is plenty of choice, so you can try it out with less of an initial investment. A range of brands and styles are available.
- Some people feel Android is easier/less confusing to use than iOS.
Pros for iOS:
- Apple Wallet is better supported for tickets than Google - but support is by no means universal for either, and you don't need a wallet app to display them, most people just show in the train company's app or as a PDF.
- If you already have Apple devices (e.g. an iPad or Mac) or use Apple services such as iCloud (which is much better than Google Drive on Android for backup/sync!) or Apple Music, it integrates nicely with those
- Some people feel iOS is easier/less confusing to use than Android.
Sites like Realtimetrains, as well as train company apps, work fine on either and you won't really be able to tell the difference.
One thing worth nothing is that if you choose to purchase apps on one platform and then you want to switch to the other one, you'll have to buy them again - whereas if you switch to another phone with the same OS (even if by a different manufacturer in the case of Android) you can keep them.
Also, some Android manufacturers have a reputation for not providing software updates for as long as Apple. This can be avoided by buying a phone which runs "Android One". Samsung are reasonably good now as well.