If you want to "restore" SMS messages from a prior backup, you don't need to pay for any software. OpenSSH + any AFC2 client is all you need. Every time someone links (legitimate or spam) to a piece of software that can recover/decode your SMS/Text messages - the tool always turns out to be - read-only (ie it can read your text messages from the phone or from a backup).
It's technically much harder to restore backed up SMS messages back to your phone (requires either a jailbroken phone or inside knowledge of Apple's iPhone backup/restore format/protocol). This is why you need to restore the SMS db by hand.
If you want to restore your SMS messages to an iPhone via iTunes, your only real option is to restore from an existing backup. It sounds like you want to backup your phone, reset to factory settings, and then ideally put just the texts back on the device. As Olethros mentions, the only way to selectively stick stuff back onto the phone (like, just the database of sms messages) is if your phone is jailbroken.
As for the trial vs full version of Decipher TextMessage - to be able to see your entire history of texts, you'll need the full version. The trial version will show you three messages per contact, to help you see that the software was successful in reading your backup files and the messages are really there.
While I'm here, and you're thinking about resetting your iPhone, it's probably a good time for me to plug a blog post I wrote about backing up your data from your phone: Decipher Media: iTunes Sync vs Backup -- How to Ensure You Keep Your Data It goes over the different ways your iTunes communicates with the iPhone/iTouch/iPad (backup, sync, transfer purchases), so you can make sure that you've gotten all of your data off your phone before you wipe it.
Good luck! I remember wiping my 3G after iOS4 came out. It was traumatizing to start over, but it did fix the problems I was having :)
Thread closed as the questions have been answered and now is just attracting spam