Personally, I would save the $$ and not buy the EBCI. Check in at exactly 24 hours, and you can sit together, even if you are in the middle of the Bs. WORST case, you would split up, 2-2. I think you would even be able to board with family boarding, depending if they do it for your flights.
Others feel differently, it is a matter of opinion.
Head for the back of the plane, of course.
One interesting thing, is how fast the boarding is, when people are choosing seats. It seems to motivate people to move faster down the aisles.