ATL is the most senior base for everything. IAD is the most junior. DFW is very senior at the top, and very junior at the bottom, but doesn't have the turnover other bases have. DTW is junior for the 700.

Newhires are placed wherever the company needs them, based on attrition. It's a crap shoot. The company doesn't run regular position bids (most companies run them monthly) so base jumping can be difficult. Also they tend to seat lock ("freeze") newhires for 1 year. Freezes are only waived when it benefits the company. Anyone who gets ATL as a newhire can expect 4 years on reserve. Upgrade is still running about 8-9 years. As -200s leave (and they will), ATL will continue to get more senior, and upgrade time will keep rising.