Not many airlines have ordered it with the full EFIS suite, but the MD-90 and 717 could be ordered with the same cockpits. Delta chose the MD-88 style system for it's MD-90s, though. I'm not certain, but I believe only Saudi Arabia ordered it. Still, retraining times for pilots are lowest with the MD-88/MD-90, a little higher with the other MD-80s and DC-9. and highest for pilots without any DC-9 type rating. I'm talking about days worth of time.
While Delta would have to pay it's pilots more to fly the 717 than Comair would (of course, the 717 is a bigger airplane anyway), it's revenue potential is higher. Passengers expect to pay less when flying on a commuter airline like Comair than on a major airline like Delta.
The 717 is very efficient, has great performance, is highly popular with passengers, has windows you can actually see out of, has lower maintenance costs, and has both limited cockpit and parts commonality with Delta's 150 (or so) strong MD-88/MD-90 fleet.