It is better to waterproof a basement from the inside. If you waterproof the basement from the outside, it will only be a temporary solution. The water will eventually find its way in, and when it does, you'll have to deal with even more damage. If you waterproof the basement from the inside, on the other hand, you'll permanently solve the problem. Not only that, but you'll also avoid any potential damage that may be caused by water infiltration.

It depends on the material used to waterproof the basement. If an impermeable membrane is used, it should be installed from outside so that water cannot penetrate behind the membrane and damage the building's foundation. If a less-than-impermeable membrane is used, it can be installed from inside, but it is important to make sure that all seams are taped and caulked to prevent water infiltration.