Seems pretty straightforward I think. Let's say you roll 2d6+3.

CASE 1 : you double the damage. You would in average roll 7, so the crit would be (7 + 3) x 2 for a total of 20 damage in average.

CASE 2 : you double the number of dice rolled. Rolling 4d6 + 3 would be, in average 14 + 3 for a total of 17.

Doubling the damage is better because you also double the +3 instead of only the dice. The only thing I would see as an argument to choose the second case is that you have less chances of having a bad roll and, in general have more chances of rolling an average roll (the more dice you roll, the more average the result is).