Google have recently improved their transit navigation offering and I've now had a chance to compare the iPhone and Android offerings.
On iPhone you can plan a journey (Directions in Maps and then choose the Transit option if available). Here you get clock buttons which allow you to shift the trip time. This is necessary because depending on your location accuracy your trip will be prefixed by a walk and Google's walking speed is very conservative. You also have to manually move forward through the steps to follow your journey. One nice touch is that where the navigations have timings in them, they are relative and live updating. So you can see how long remains until you should reach the next leg in your journey.
On Android you get full blown navigation with the Navigation app. The live updating of the directions list in Android is different. It is static text but the GPS moves down the list of stations. The time and distance remaining on the current leg is shown when the GPS is available.
I wonder, given that the London Underground was added to Transit last week how well thus works because on Android it relies on GPS. I didn't get updates in the only tunnel I go through.
Really good for free on both if you have good Transit coverage in your area.