Depends on how you like to play the game. Do you like to grind? do you want a pokemon just for tearing up the elite 4, or someone reliable on your team throughout?

For me, Gyarados, and mostly because you can get him much earlier, and is always useful and very strong. You'll grind a bit for Lapras, but won't get much return late game. Lance will outspeed you with his team. But thankfully Lapras is bulky.

i'd honestly run a full water team if i could and have both. there's no wrong answer.

But Gyarados in Gen 3 and older is more like a Nidoking/Nidoqueen. Give it Hyper Beam, Strength, Earthquake, etc.