I would say that it would depend on the type of location for the pictures you take. I have both lenses and the 100mm gives you the same image as the 60mm, but at a grater distance to the subject. Here in norway we are not always blessed with great visibility and then 60mm is great, but you need to get closer to the subject to get full magnification. Heres where the 100mm is great as you will get the same image but you do not need to get as close and this makes it easier to set up your strobes to avoid backscatter and you also are not scaring the wildlife as easy.
I use focus rings to acheve manual focus, but the lens is always set to auto focus - i use the manual focus to assist the camera finding the focus point.