This depends on how much of a gambler you are. If you're interested in consistent average damage, you're better off with 2d6. If you're looking to roll a 12, you're better off with 1d12. You can't roll a 1 with 2d6, but you're also less likely to roll a 2 with 2d6 than a 1 with 1d12. In short, 2d6 will give you consistent middle numbers, while 1d12 is a bit more wildcard.