I'll assume for the moment that you have absolutely no landscape design, material, or planting experience.
You might be able to do just as well with a landscape designer as a landscape architect. The LA will have all the credentials from schooling and testing for state registration (for states that recognize LA as a profession deserving such status...and not all do). But many times, and my experience shows me this, the LA with all the credentials may not have the best "hands-on" experience. The landscape designer may have more of that, maybe not. But the question is very much worth asking if you take the path to hiring someone. Ask the questions, and ascertain just how much "field experience" your designer has. It can make all the difference in the world. And ask how much experience the designer or LA has with local nurseries and availability of plant material. The last thing you need is someone designing for material that can't be located in your area. And it does happen. So make sure you know how much local, hands-on, field experience, and knowledge of local nurseries and availability your designer has.
[This message has been edited by treebeard (edited June 08, 2003).]