Digital Foundry reports that the resolution has changed between the Xbox 360 and PlayStation 3 games. Sigma runs at 1280x718, effectively 720p, while the Xbox version ran at 1120x585. The resolution boost comes with some cuts: DF says the PS3 version loses some polygons, or rather some whole collections of polygons: Sigma 2 maintains a limit of six enemies or fewer in most situations, while Ninja Gaiden 2 "revels in throwing seven or eight opponents at you at any given point." Things that didn't get cut: The lighting effects, which are noticeably more advanced in the PlayStation 3 game.
DF also has a video demonstrating a stress test of sorts (forcing as many enemies onto the screen as possible). For some reason, we don't mind the idea of having slightly fewer ninja enemies stabbing us in the back at any time. These games are at their best when Ryu is juggling a few enemies, not when he's buried in them.