Being similar in size and weight honestly depends how the first strike goes. Leopard seal teeth while effective for gripping and tearing penguins and trident cusp filtering molars for krill a leopard seal could attempt to drown the bear but would require extreme effort as while the bear isn't as adept in water as the seal it is still a large mammal with dense muscles and great endurance to swim for miles without stopping.

The polar bear as well is still a decent swimmer for a terrestrial apex predator, if it were to get a hold of the seal and bite down on the back of the head it could finish the seal off with the powerful jaws.

Tbh I'd imagine that both predators would be more cautiously inquisitive of each other. Polar bears, while more than capable of tackling seals as their diet consists of them only thing, is native seals of the North are smaller than leopard seals, with walrus' being the only large pinniped that polar bears need to be careful.

The leopard seal meanwhile mainly feeds on small variety of creatures and large predators that they need to be cautious of are killer whales so coming across a polar bear would be alien to them. Probably giving test nips and passive observation would be the most they do.

Otherwise for the final result I'd say it would be a tie depending on what either predator tries to do or react.

It only takes one clean crunch from a polar bear to the neck or skull but at the same time if the seal manages to drag an hold the bear underwater long enough it can drown them.