If you're phone is not jailbroken then you have to use the stock SMS app. This also means that no apps in the app store have access to your SMS messages. Weather you see this as a drawback or a benefit is a matter of opinion, but I actually like the fact that I don't have to worry about 3rd party apps accessing my SMS messages
There are loads of apps on the Appstore that are used for messaging BiteSMS and Whatsapp to name 2. Not aware of any apps jailbroken or not that have access to your messages anyway. The only security risks associated with the iPhone is location tracking and being able to access the phone by bypassing the lock screen.