Well both are extremly good Pokemon, each a water type so I understand this dillema.
Well IMO lapras would be a smarter choice, but I'd choose Gyarados anyway cause I love it!
It really depends on what your team needs... If you need a good sp.attack with a diverse movepool who can take a hit, I'd go with Lapras!
If you need a hard hitter, but very fragile to electric attacks, then Gyarados is your choice...
Since this is an in-game question, I reccomend Lapras since it's easier to lvl.up!