ClubFootedPilot--NYC is an easy commute from DCA, so long as you report after 0900 and release NLT 1700. The shuttle only flies 6am to 7pm, so if your report or release times are outside of those windows, you're spending a night in NYC. As a new guy, you'll be on reserve, and so you'll have to get a crashpad or plan on spending a lot of money on hotels. That being said, you'll get seniority quicker in New York, probably. Even after you get enough seniority to hold a line, your trips will most likely be uncommutable on at least one end. Very early reports (0500 or 0530) or late releases (2200 or later) are common in NY.

Pay attention to what airports your desired aircraft typically departs from. Remember that the NY base covers all three NY airports, LGA, JFK and EWR. Getting around between airports in NY without a car either takes a fair bit of time via public transportation or a large amount of money (esp. from NY to Newark). You won't have this problem in Atlanta.

There are lots of flights every day to ATL from DCA, and several from IAD. An additional benefit is that you have very late flights from ATL back up to DC. This gives you additional flexibility in bidding for commutable trips. You can finish pretty late in ATL and still have time to catch the last flight up to DC. The same is true of DTW, though there are fewer flights between DC and DTW. ATL-DC flights are nearly always full, so you'll be in the jumpseat a lot.

There are always seats on the shuttle to NY. In 2 1/2 years of DC to NY commuting, I have not made it on only one flight, which meant waiting until the next one an hour later. I took the train home once because weather cancelled all the shuttle flights.

I commute DC to NY, currently on the 7ER so I have to get to JFK. Weekdays there are only 4 flights to JFK from DCA, and they're regionals, so no jumpseat reservations. These flights nearly always go out full, but also have lots of no show passengers. Cabs back and forth between LGA and JFK run $20-35, and you can nearly always find someone to split that with you, esp. from LGA to JFK.

I still get some noncommutable (on one end) trips in NY. I have a free place to stay up there, and keep an airport car in NY. If I had to get a crashpad, or pay for hotels, I'd be looking hard at DTW or ATL instead.

Your circumstances may vary.