The major feature that has come to both platforms is a new option which will automatically sync the read state of Direct Messages, so if you read a direct message on one device, it will automatically be marked read in all other official Twitter apps on other devices. This is definitely a nice feature, and one we hope will filter out to third-party apps as well.
Both platforms are also getting an enhanced search experience which will show when new tweets match your search terms, but the iOS version has gotten more changes than the Android version other than that main update. iOS users will also get a new reply compose experience, a new People button on the navigation bar, and an option to report unwanted tweets via the More menu. Android users will see user avatars in search results and an option to clear past searches.
If you want to grab the official Twitter app, it is available for free in the Google Play Store and iTunes App Store.