Quite a few features have been brought to each. The major feature I'd like to highlight is that iMessage now truly synchronizes across all devices. Previously the iPhone was the only device capable of receiving messages sent to your phone number; messages could be synchronized across all devices if they were all set to use an email address for iMessage, but this lead to issues of its own. Now your computer and iPad can receive messages and FaceTime requests that are routed to your phone number.
