You know, I DID buy a Pain Of Salvation cd, and it was wimpy. I once put the Pain Of Salvation cd next to my Slayer collection by mistake, when I came back the POS cd was smashed beyond all recognition and the photos of all the people in the POS booklet somehow had frowny faces on them and were crying.
The box set is awesome Blitzy, the first 2 cds are basically a "best of" along with the japanese bonus tracks and the soundtrack releases Slayer has done. The third cd is rare stuff. The demo for Raining Blood is awesome, it sounds nothing like the original at all, and theres no vocals, Jeff just solos over it (and solos properly showing he can play if he wants). The South Of Heaven demo is cool as well, the main riff has a weird echo effect on it, its way creepier
Plus the riffs are all different for the most part, Jeff must rewrite things heaps. The DVD is quite good as well, some of its varying quality (the early stuff) but its interesting if you are into Slayer
I dont know what the price difference is, but the 5 disc set was worth it to me, you get an extra live cd in a "blood pack" with floating skulls in it, a huge wall banner, different packaging and I cant remember what else. The book that comes with it is great too, some classic shots from their glam era