I've had both with RB transplants. A-body was definitely a tighter fit, plug changes were more of a hassle, the '63 & '64 were surprisingly tight on the right side compared to the later Bs.
A has a little more room in front than early B, unless you set back the motor & trans with later K-frame and modified trans mount, which also really improves weight distribution.
Both bodies are kinda nose-heavy, B has the longer wheelbase of course but they seem to hit the rear tires pretty well. A-body's easier to pitch-rotate on big tires.
Early Bs have more rear tire room than Darts by a significant amount.
The early Bs can certainly run Nostalgia classes because of the Maxie thing, not sure about a '69 Dart.
Early B body interior/exterior trim is harder to find.
Me, I'd take the early B.