I think one difference is the thickness of the rice paper that’s used. Lumpia wrappers come in a dried form and have to be soaked in warm water before they’re pliable enough to use. They’re so thin they’re almost transluscent. The more traditional American-style Chinese egg rolls use a thicker rice-based wrapper that’s about as thick as a tortilla. They’re pliable and ready to use right out of the package.
I’ve had a hard time finding "egg rolls" in Taiwan and Hong Kong, which makes me think that they are more of an American creation made by enterprising Chinese immigrants, or maybe they were something that went out of style in Asia but stayed alive over here?
I’ve also noticed a difference in the fillings. Lumpias tend to go lighter on the cabbage and have more interesting stuff, like shredded carrots, scallions, and noodles.