My issue with drum brakes is that I feel they trap brake dust in, so you need to clean them periodically (more than disc brakes). Plus, disc brakes (in my opinion) give you better stopping power because you're grabbing a moving disc, rather than trying to rub against a moving surface.
I think the disc brakes will lock easier, but with 4 wheel ABS, this shouldn't be much of an issue. Personally, I haven't felt a big difference between my 2010 Focus (which had rear drums, but 4 wheel ABS) and my 2012 Focus which has all 4 disc brakes with ABS.
But if I had to choose, I would always take disc breaks for the rear over drums. They're also easier to work on than drum brakes. And you can make better visual inspections with disc brakes, instead of having to remove the wheel and drum just to check the status of your pads.
2012 Focus SE Hatchback
SE Sport & Winter Packages