As a designer, I am somewhat biased, but one problem with software
packages is they group plants by zone. Plants that do well in the
east zone 7 will not necessarilly survive in southwest zone 7. Too
many variables in plant selection alone. A designer ( a good one
anyway) should take as much input as possible from you on desires of
look, colors, etc and use his/her experience, knowledge, and talent to
give you the look you want using what works in your area and your
particular site peculiarities. If you go with a designer, I suggest
that before you meet with one, looking at other landscapes and
magazines and collecting pictures that show the type of look you like,
dont worry so much about plant types, just the look you want. This is
very helpful to a designer in understanding your tastes.

To do it your self, I suggest learning as much about what works in
your area as possible before trying to plan anything out. I can see
the software being helpful if you already have a plant selection based
on your area recommendations, but cant picture how it looks together.
Just dont trust the software plant list. I agree with the previous
poster who said it is quicker and easier to do it by hand, you just
have to be able to picture the plants in your mind. And as others
said, dont be in a hurry, it is too expensive and too much of a long
term investment to rush into.

Good luck,
Bryan