My 2 cents...
Android and iOS differ enough to mean the Platform learning curve will generally be similar between cross-platform and native. After that it's down to language learning curve. If you can pick up languages easily (or enjoy doing so) Swift and Kotlin are both languages I've had a lot of fun working with. However if that doesn't apply to you and/or you're already familiar with the C#/.net world, Xamarin's a good option for getting up and running a bit faster.
From my own skill/knowledge development perspective Swift has been valuable to me in introducing a number of concepts and design patterns that weren't popular (or sometimes even possible), in my previous language experience. In that sense it's been a valuable language for me regardless of its utility in Apple platforms development. Probably not a surprise that someone on the Swift forum would recommend learning Swift...but here we are