Unibody is less expensive and doesn't weigh as much. But with out the frame to tie front and rear together, the body is flexible. Handling and putting power to the ground can be inconsistant. The more it gets flexed deterioration can take place making the flex even worse.
With a chassis just the opposite. Though if you flex the thing deterioration can take place here too.
I wouldn't reccomend a unibody if you plan to drive in a sporting manner. I've seen late model mustangs with the rear control arm attachment points ripped right out of the body from launching really hard. Subframe connectors are madatory on a unibody IMO.