We were making an app for booking quest rooms. At first we thought of going to both platforms at once, but the budget didn't allow it. In the end, we chose Android, simply because most of our clients were Android users. Then we added iOS when it became clear that the idea was working. By the way, I advise you to take a look at A-Bots.com - they have a good description of what to base your choice of platform on. We worked with them - they helped us to clearly lay everything out in stages. The main thing is to look at where your target audience is.