1.7 is functional and really fast and more secure. I work with 1.7 every day since last 2-3 years in online shop and offline shop too because we sell with prestashop in offline store too... So I am really happy with 1.7 version to go back to 1.6 to use one module or another.
1.6 have interesting vulnerabilities... You can use some when you want to know delicate information from other shops
Be carefull and do backups always!
Is it the latest version?