Actually, for both NVIDIA and ATI cards, hardware acceleration isn't as consistent as software mode in terms of performance for both OS 9 and OS X. Hardware rendering may seem high at first, but once a lot of stuff starts happening on screen then performance drops through the floor.
The absolutely best way to play Diablo II is with a 3dfx card (Voodoo3 or greater) using Glide acceleration. The 3d accelerated aspect of the game was designed with Glide in mind because at the time 3dfx was the king of 3d game acceleration.
In OS 9, RAVE is alright, but heavy traffic can cause major issues. With OpenGL, performance is even worse.
In OS X, OpenGL is the only hardware accelerated API to choose from, but there are issues with texture loading and unloading which cause major performance hits (according to more than one Mac developer).
With FPS games, most textues load during level loading before any action starts. With a game like Diablo II, which is basically a 2D game with a bunch of textures, textures are constantly being loaded and unloaded from the 3D card's VRAM. Because the maps are so big in Diablo II, it's not feasible to load all the textures for the game within the VRAM, let alone the system RAM. Since OS X's OpenGL has slow texture loading and unloading, Diablo II performance suffers.
So basically, unless you're running in OS 9 with a 3dfx card, I think it's better for people to use software rendering. Even with my dual 1.33 GHz Mac with a Radeon 8500, I use software mode in both OS 9 and OS X because of the very noticeable performance issues and even major graphical glitches which occur with RAVE and OpenGL rendering.