Side radiator, draws air in from the side of the coach , blows it across the engine and out the grill to the rear.
Rear rad pulls up air from ground level under the coach and blows it out the rear of the coach.
In both cases really harmful debris would have to be small enough to pass through the rad before it gets to the toad, so I can't see any difference.
Majority of damage , to the toad, will come from stuff thrown up by the dual wheels , that manages to bounce around long enough to get that far back.
Don't now if any one has really studied this , but I can't imagine the difference would be dramatic.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.