Laine is a wonderful goal scorer, and does some things better than Matthews (such as the Kovalchuk/Ovechkin/Stamkos style one-timer on the PP), but it's hard to put him outright better than Matthews.
Over the past two seasons (plus the beginning of this season) of even-strength play, Matthews scores goals at an astounding 1.68 per 60 minutes played. Laine is at 1.24.
On the PP, both are absurdly good, with Laine clocking in at 4.12 goals per 60 to Matthews's 3.02. It evens out somewhat when you look at primary point contribution, though, where they are basically the same (5.07 p1/60 to 5.03).
To sum, Laine is a better PP goal scorer, and Matthews is a better ES goal scorer. And Matthews's primary point contribution on the PP really negates his advantage there, and primary assist+goal scoring rates on the PP probably reflect more Leafs PP1 shooting options.
There is really no strong argument to make that Laine is a better overall goal scorer, and plenty of evidence to suggest Matthews is a much better overall player.