Depends what kind of photos your father likes to take. If he loves portraits with a very blurred background, then yep, the f2.8 models will be better in this respect.
That said, the Canon can zoom-in closer, so can still achieve a blurred background at 85mm f5.6.
See the second photo in this gallery for an example:
http://www.cameralabs.com/features/Cano ... age5.shtml
I'd personally go for the Canon because of the longer range, image stabilisation and USM focusing.
PS - to see what you can do at 55mm f2.8, see the second photo in this gallery:
http://www.cameralabs.com/reviews/Canon ... age5.shtml
For the ultimate bokeh though, why not go for the 50mm f1.8 or f1.4? Affordable and very very bright!