Is it better to waterproof a basement from the outside or the inside?
When applied correctly, I prefer waterproofing from the outside.  It keeps the water out of the concrete, rather than permeating up to the interior water proof layer. If it's allowed to freeze in the foundation, you could have cracking.

In addition to water proofing the foundation, you also want a weeping tile or French drain around the foundation anywhere that water may not naturally run away from the home.

That said, as you're seeing, fixing this issue from the outside can be much more difficult, and therefore expensive. Another factor is if you need to use a sump pump to move water from any drains up and away from the home. If that's the case, you also need to consider if you want this pump indoors or outdoors.