Trying to find some peer-reviewed article of book about the subject, found the "Manual of Veterinary Dietetics", from Saunders / Elsevier.
The conclusion is that Soft / Hard foods don't have any clear winner. Many studies were made /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 it that same breed 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.