Trying to find some peer-reviewed article or book about the subject, I found "The Manual of Veterinary Dietetics," from Saunders / Elsevier.
The conclusion is that Soft vs Hard foods don't have any clear winner. Many studies were made or reviewed, and the authors found it hard to compare the conclusions because each study involved different animals, with different methods of analysis.
Those authors' conclusion was that animals of the same breed may have more tendency on developing dental problems since their anatomy is different, for example, having more or less space among the teeth.
Some special food (textured dry food), on the other hand, could be beneficial in the plaque removing process, and flat rawhide chews of small sizes could help too.
MANUAL OF VETERINARY DIETETICS 0-7216-0123-5
Copyright 2004, Elsevier (USA). All rights reserved.
Authors: Tony Buffington, Cheryl Holloway and Sarah Abood.