Delta lets you pick your seats for no charge.
I can tell you from experience, all of the Delta flights are not in the same area of the airport. There is actually a subway that connects the seperate terminals. If you have to change terminals, take the subway. Do not try and just use the moving sidewalk, the subway is much faster.
Something else to consider is how much traffic goes in and out of Atlanta. We have flown into Atlanta before, and circled quite a while before landing. Also, if you are farther back in the airplane, it can take a bit of time to get off, and even get on your way.
A few years back we did a 47 minute layover in Atlanta on the way to Orlando. This was on Delta as well. Our first flight landed 10 minutes early. We got off the plane, and high-tailed it to our next gate, which was a couple terminals over. No stop for bathroom, or drinks, nothing. We had 5 minutes before they started boarding the plane.If our plane had landed on time they would have already been boarding by the time we hit the gate. If we would have been late, we would have been in a heap of trouble. The youngest in our group was 7 or 8 at the time, and had flown many times. I pretty much dragged her thru the airport.
As far as holding the planes, sometimes they do. But not always, and you cannot count on it. It depends on things such as how many people are missing on that flight, and how urgently they need to get the plane to the next destination. I've been on planes that were held for missing people. I've also been told my connection left, and I had to sit in the airport for 9 hours before getting on another flight.
Forty five minues is doable. But it is not easy, and everything needs to go just right. If your first plane is late, or you are stuck at the back of it behind some slow moving people, all bets are off. It is a risk. We have done it, but I wouldn't recommend it.