It has a full ring-type USM autofocusing motor, instead of the inferior micro-USM motor in the 50 1.4. Faster, quieter and more accurate autofocus in 85mm and sharper image quality. Main downside is heavier purple fringing, though anyone buying this lens should have lightroom or its analogue, which removes fringing with one click.
I think the more significant difference between the two is the focal length however. If you're primarily shooting inside and prefer to shoot full body portraits, the 50mm will be more suited to that. If you're shooting outside or inside in a large space, or prefer tight portraits, the 85mm is better in my opinion. This is assuming you're using crop-body cameras (e.g. Rebel T4i, 60D, 7D), and not full frame.
Last edited by Sindarin; 04-02-2013 at 05:07 PM..