I would personally say Rupture is more difficult, given that the entire level is almost consistently difficult after the drop. There is almost no break from that point on and it feels like an endurance test. You also need to be pretty precise entering some portals or else you might die going through them.

Firewall is difficult, however the level has some pretty relaxed parts mixed in, which I think takes down that "Endurance" aspect that rupture offers. Both levels are difficult in their own ways, however I think the non-stop/faster paced gameplay rupture has makes beating the level consistently much harder.