If you are going to do a lot of design work using various vendors gear, you should buy a pro tool Eakhau or AirMagnet. If it is for a single deployment many vendors have design tools built into their products. You can also go the consultant route and hire a pro that does a lot of this work. The investment up front will pay off in the long run.
The Aerohive plantool is a great tool to predict coverage by the way, but as we say in Dutch: "Meten is weten" :-)