Well, lunchbox... (or was silent bob the basis for chronic?)
The suspension will make it a little more comfortable for offroad and help keep the tires in contact with the ground, which improves handling. It can also help reduce the stress on the frame.

On the flip side, suspension parts weigh more, cost more, and will eventually wear out. Plus, learning to ride on a rigid will help you develop more skill and you will eventually learn how to use the frame's solidness to your advantage.

In your price range, a full-suspension bike is usually considered a no-no. Manufacturers like Huffy and Mongoose cut too many corners in part quality and design to produce low cost full-suspension bikes so that they aren't worth it. Decent hardtails (front suspension only) however, can be had for just a little more than the cost of a rigid. The Trek 820 and the Giant Boulder SE are the next step up from the bikes I suggested earlier and come for just under $300. The suspension forks on them aren't very impressive, but they do take the edge off.