Your internal sd card is safe. As for rom they are both quality it is a matter of preference and what you need the most out of your rom. I recommend trying both at least for a little while. Just remember to make back ups.
Yeah both are great. I'm running Calkulin now, which is also nice. I still have a Nandroid backup of my Viper setup though in case I decide to go back to it. I really liked viperROM. It's a very clean setup.