Singleplayer or multiplayer?

Singleplayer, D2 right now is so very much better, because SP in HGL is horribly horribly screwed up and 2 months of patches behind, so far. MP, they're close, but it depends on your mood... generally I prefer HGL, because it makes me feel like what I do matters more. FPS style is more immersive than just clicking every bit.