The answer is there is no way to tell if you will be seated together. Either pay the extra to select seats, or take your chances. Delta explicitly says they are under no obligation to seat you together. It's pretty black and white.

Your question of 'is it likely'? This depends on a lot of variables. If the flight is empty, yes. If the flight is full, no. Even if the flight is empty, if there are any delays or cancellations (of your flight or another flight on the route), then it suddenly becomes less likely.

Nobody here can see the future and there is not going to be enough of a sample size to give you the 'odds' on your particular flight. If you want to save the $100 I would do so under the assumption that you can't sit together, and you'll both be in undesirable seats. That way you won't be disappointed.