Having used both a green and normal pass to travel around, I'd say only go for the green pass if you're spending a lot of time on the Tokaido line or the Tokaido Shinkansen and you really want the few extra inches of space to yourself. Most trains I encountered around Tokyo and Kyoto didn't have green cars in the first place, so there was zero benefit from the more expensive pass there. Given most of my travels in Tokyo involved the Yamanote and Chuo lines, and in Kyoto the Nara and Sanin(?) lines, none of which had green cars, I wished I hadn't bothered with the more expensive pass. As others have said, there is no additional dedicated luggage space in green cars. Needless to say, that was my one and only time paying the extra for it.

That said, the green cars were way quieter and I never had any trouble finding a seat, even when the rest of the train seemed to be packed to bursting. I even managed to get a whole car to myself on the NEX into Tokyo.

On places to buy it, I had a pretty bad experience buying from Railchoice. They issued the voucher fine, but contacted me 7 months later to say they had no record of my payment and tried to charge me again. I had to scour my old credit card bills to find the transaction. Next time around I used Inside Japan Tours who didn't cause me any grief, and were significantly cheaper ^