I appreciate a good Sophie's Choice question. I feel like an uber-old motherfucker for always being the only person after the year 2000 to still use this metaphor, BTW. It's so perfect! Bring it back!!
Anyway - tough question but it didn't really take me long to get to Matrix. Everyone (esp those who were there during the times these were freshly released films) knows how impactful and important both films are. Personally I will always leans towards the Matrix since it truly caught me by surprise, was not expecting much whereas there was a fair amount of hype for T2 for reasons which shouldn't need explaining. Other than that, though.. both films were technical feats, and for that I would say T2 was more impactful in that regard overall. But story, characters, overall importance of each film culturally and philisophically, I will hand it to the Matrix. I feel like it changed cinema more profoundly, and not merely because of its effects. Just in the way of what a movie can be, particularly with that absolutely magical reveal scene. That was something that was so jarring, I haven't really seen anything like that before, and certainly not since (I'd love to hear arguments about this, inform me please!) That was just such masterful storytelling, and manipulation of the viewer. Disgust, amazement, incredulousness, all done with no dialogue. Everything that followed that scene was about as well-done as is capable in a modern film. I can go on and on about it for paragraphs so I will shut up now.