Apple Maps generally always gives fairly conservative but accurate estimations of walking times, while Google Maps is all over the place. I’ve missed perhaps half a dozen buses while using Google Maps because of unrealistic walking times, but none while using Apple Maps. On the other hand, Google Maps has also sometimes overestimated walking times by ~20 minutes.
As for using public transport, Apple Maps is just more useful. It’s easier to find stations, easier to find out about disruptions and gives better walking directions around stations. Google Maps regularly gets trains and buses mixed up, makes it a pain to find information about disruptions, and in a city with multiple modes of transport (i.e. Melbourne AU with trains, trams and buses) a cluster of train/bus/tram stations on a map just gets rendered as an unreadable mess.
Google’s UI is also a bit of a pain. Several friends (both iOS and Android users) have complained that Google Maps is too fiddly to use on the go. I’d agree, and add that Apple Maps is a lot easier to use on a crowded train or while walking.
That said, for a long time I persistently had problems with Apple Maps being slower to load actual maps. I suppose I’d better start collecting my own usage data and write a blog post.