They are both very close to each other in terms of pure damage output. Technically Ice Beam is about 7% better however. That is because on defense you want to be casting as many charge moves as possible. On average Ice Beam Lapras ends up spending more time on using charge moves thus resulting in higher overall damage output.
That is similar to Water Pulse Vaporeon dealing more damage than Hydro Pump Vaporeon even though on paper it does indeed seem the opposite. Energy is also generated from taking damage, so often times you see Water Pulse Vaporeon just spamming charge attack after a little while. It's a little hard to explain but I'm sure you'll get a grasp of it once you put some thought into it.
Here's the most accurate spreadsheet currently known to public. If you go to Results_Simple tab you can compare all movesets with each other. https://drive.google.com/file/d/0B0TeYGBPiuzaYWhMNV9seWs1emc/view
