Crank bearings. Much easier job. With worn piston rings you have to not only take out the crank, but the entire rotating assembly just to get to the rings to replace them, as well as the heads and all other associated top end parts just to tap the pistons back in. It's a complete teardown. Not to mention, if the piston rings are worn out, more than likely the cylinder walls will be worn/tapered too, which would require the block to be sent out for machining to be fully cleaned and honed. With worn crank bearings, you can just take the crank out with the rotating assembly still in the cylinders to replace the bearings and replace the crank if need be (if it has worn/damaged main or rod journals). However, if there are metal shavings in the engine, it would more than likely have to be completely rebuilt. But I'd still say crank bearings.
Skating on a rink or a frozen pond?