There is no good answer for this question, unfortunately.
With Windows 10 for mobile, once it is available, Microsoft is making it easier than ever for Android developers to port their apps directly over to Windows phone, and in many cases without even having to make changes to the app. In theory, it should make the app gap non-existent, but in practice no one knows what will truly happen until it happens.
We'll just have to wait and see what happens.