You can't really compare the two. Both have similar stats, almost the same BST (Lapras with 535, Walrein with 530), both have the same typing, but Lapras learns more Water Moves, and Walrein learns more Ice Moves.
Honestly, it all comes down to preference and your team. Need a water-move user? Go Lapras. Need an ice-move user? Go Walrein.
Walrein is much better in Hail, thanks to Ice Body.