The difference is not all that great, but it is a little bit better to buy the 12 pack of 355 mL cans or bottles instead of the 2 liter bottle.

What you have to do to find this out is first to find out how much soda you are getting in the cans. 3*355 makes 4260. So you are getting 4260 mL of soda in the cans. Divide that by 3.99 and we find that you will get 1067 mL per dollar at that price.

Now divide 2000 (mL in 2 liters) by 1.99. There, you only get 1005 mL per dollar.

So by buying the cans you get an extra 62 mL for each dollar you spend.