While in Fiji, I read the PADI book cover to cover, we then fired the only PADI instructor on the island (Well, he did break the jaw of our Dive Ops Mgr. Fijians, eh?). I then had to go for the SSI, I was disappointed but it got me into some wonderful dives.
Reading the PADI book then the SSI book, I'd say the PADI course definitely prepares you for diving in much more detail. I somehow feel like I got the "economy" version. Definitely want to go over the material again.