Do you have a specific resort or category in mind? I like booking by phone. The CMs are generally very helpful. They can give you information that takes a while to get to online - for example, what resorts still have free dining package rooms in the category you want, or compare prices across categories, for example if you want to compare a moderate and deluxe. On-line will give you a couple of "offers" for alternative resorts, so if you aren't sure what you want, you could see what is offered on-line first. I found that the price was the same.
I always book online -- it gives me more control over my reservation and I don't have to worry about being pressured into things I don't want. I've heard too many stories about errors being made when you involve other people. Online the only "error" would be one made by me.
Now if they would only allow you to modify a package online I would be much happier.
For our trip next year I booked on line. All other trips I called and booked by phone. I found it very easy to book on line, but I have alreay called to change our dates around. Any changes after your booking need to be done by phone.