Most traditional pick-up trucks and SUVs were frame-on-frame. However, in terms of ride comfort and handling, having a uni-body is great. However, I wouldn't really go major off-roading in a uni-body car. Each design has its pros and cons.

If I was really going off-road and/or hauling lots of stuff in a pick-up bed, then a frame-on-frame can be more durable.

However, if your AWD SUV is mainly for street use and mild rough road/off-road use, unibody will serve a much better platform for handling and ride. Thus, BMW X5 and Lexus RX300-330 has unibody. The original ML was a frame-on-frame, but most people didn't take their MLs off-roading. However, the on-street handling and ride is abominable when compared with the X5. The new generation ML is built like the X5 (geez, Mercedes have been copying BMWs lately with i-Drive and stalk control for auto tranny, etc.)

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