Google Now and Siri and two very different things at this point in time. Even though there is some overlap, it's rather silly to compare them. Google Now is mostly about providing the user with data, via cards or voice queries. Siri more focused on interacting with your device. I use Siri constantly for reminders, appointments, timers, alarms etc.
Siri keeps me organized, Google now keeps me informed.
For the record as an American with very little accent, Apples dictation is a hair better. However I so find that Apple struggles if I'm not using the built on iPhone mic. For example it can't understand me at all through my cars Bluetooth.