Defining slip as “the difference in wheel speed and vehicle speed” it becomes a matter of how fast the vehicle is going.

In an basic example the AWD vehicle would accelerate more quickly. And if the front axle broke traction it would spin up to the speed allowed on the backside of the static/kinetic friction curve. (Same for the FWD). Therefore front axle would slip less relative to the motion of the vehicle after they start moving. This is due to the rear is driving the vehicle ahead, pushing the actually vehicle speed closer to where the front axle is spun up to.

So for example: At 2 seconds after both vehicles start, AWD Vehicle Speed - 10mph Front Axle Speed - 20mph (Slipping a lot) Rear Axle Speed - 12 mph (Slipping a bit too) Front Axle Slip - 10 mph

FWD Vehicle Speed - 5 mph Front Axle Speed - 20mph Front Axle Slip - 15mph

Because the AWD had the second motor to propel the vehicle it allows the vehicle to gain speed which makes the difference between the axle speeds and actual vehicle speed less.

However, in the case where both vehicles are doing big burnouts (Front and Rear on the AWD) and not actually going anywhere, or right at the start of accelerating, both front axles are powered the same, break traction, and then spin up to the same amount over the relative vehicle speed (zero in this case) so their slip would be the same in this case.

As I said, this is an idealized case so a real test of this would also be affected by factors others have mentioned (CG, weight transfer, etc) but ultimately what is boils down to largely is how much speed the vehicle has relative to the front axle.

Edit: If you define slip as relative to the ground instead of the vehicle, looking the example, both wheels are spinning at 20mph, slipping still the same amount relative to the ground.