Water moves are special in Gen 2, and Gyarados has a really bad special attack stat. Lapras would be able to make more use of Water moves due to having a higher special attack, and it also gets STAB from Ice moves too. That makes it helpful for dealing with Lance, or basically anyone who uses Flying or Dragon types.
Lapras can also make use of physical moves as well, so you don't have to worry about being stuck with a mostly Water/Ice moveset. It also gets some pretty good TM moves.
You also get Lapras waaaay before the shiny Gyarados (and if you used a regular one instead, Magikarp is still a pain to train up) so that's another good thing.
So yeah, I'd say go with Lapras.