It seems AD is on the road to replacing Civil Design, but as of yet, Civil 3D is not an all inclusive drafting program. There are a lot of really great things in Civil 3D but it is not as complete as a LDD/Civil Design/Survey setup. Fortunately, a regular seat of Civil 3D comes with a complete LDD, so you won't lose any productivity and Civil 3D Pro comes with LDD AND Civil Design. If you want to keep using LDD/Civil while transitioning into Civil 3D (which, if they ever get the buggies squashed, is one hell of a program as is and as they're working on adding some of the missing bits to the next release, can only -hopefully - get better), I suggest Civil 3D Pro. Trying to add Civil Design later ends up costing WAY more than having them all arrive neatly packaged on your doorstep.
Oh and test drives ain't just for cars anymore. You can call your local reseller and ask for a free 30 day trial of Civil 3D.
Hope this helps!