I have not flown Delta's BE, but from what I have seen at the gate the seats assigned are very random. There is no guarantee that you'd be seated together, and your odds of being stuck in a middle seat are good.

If it were me, I'd pay the extra $100 and pick my seats. That's just me. I know that's not an option for everyone. But I do have one question - when are you flying and what's your departing airport? Sometimes those can make a difference in the fare, and also fares do vary depending on what day of the week you're flying. Sometimes shifting the dates just a day or two one way or the other (if that is an option) can make a difference in the pricing.