If you dont care so much about 3d audio, than I'd say theyre on about equal footing.
I've used both the Audigy/Audigy 2 and Sounstorm extensively. Here's the bottom line.
Audigy: Slow performance, pretty good sound, pretty damn good 3d sound, eax advanced HD isnt used in enough games to make it matter, but it works properly in regular eax games.
Audigy2: Same as above, except better DAC quality and better performance. It is a pain, but if you get the audigy2 drivers to work on the audigy 1, the only difference between the two will be the DAC, and its honeslty not too noticble. Performance is nothing to complain about.
Nforce: Performance is great, quality is great, but in all honestly, this thing has SERIOUS problems with 3d sound. Practically every game I play comes out wrong. Doesnt support eax advanced HD. Barely works with regular EAX games. The reverb is usually WAY too strong. In counter strike, there is a driver bug that puts a major echo on everything in 3d mode, even when eax is supposedly off. Nvidia uses an algorithm that cuts normal sounds in half, so it can double the volume of sounds closer to you, which is supposed to make it sound like the source is right by your ears. It doesnt really work in reality, and you just have to lower the music in every game to achieve some sort of balance. 3d sound is totally screwed in unreal tournament as well. Same with most every game that uses EAX.
Personally I still use the nforce, and if 3d sound doesnt work properly as usual, I just turn it off. Its not worth 50-100 bucks to me to have 3d sound in games, especially when it seems like a dying trend. If youre down to spend some dough, get the audigy 1 and hack the audigy 2 drivers on it. Its worth it. If you absolutely must have the superior dacs of the audigy 2, go for it, but dont do it for performance because the same can be acheived with a little work on an A1.