It seems lately, that all bases and equipment are possible for new hires. It just depends what they need at the time.

If you get the "opportunity" to bid:
ATL: if you don't live here, avoid it. You will spend a LONG time on reserve. Both 200 and 700/900. ATL 700/900 is super senior. I expect to see ATL shrink as DAL acquires 717s, especially in the -200 categories.
IAD/DFW: small bases, way senior at the top, way junior at the bottom. -200s only. Low-medium time on reserve. Not very commutable.
DTW: All -700/900s. Decent commutability from everywhere except ATL. Least time on reserve. Long legs, not many legs per day, flying to most of the large airports in the midwest and northeast. I expect DTW to see the most growth in the near term.