...same discount, I agree. However, if you buy today, you get to pay today's price. If you wait until you go to purchase (I don't know how soon you're going), the price may have increased... Not that I know when the increases are coming. But this is Disney, so they *eventually* will be arriving.
You only get the DVC discount if you have your DVC card with you when you go to Guest Relations. This applies whether you have purchased the vouchers in advance or are buying the APs on the spot.
Both times I've done this I fortunately had my DVC card in my wallet, but had completely forgotten that I would need it!
I'm not sure what they do to you if you purchased DVC-discounted vouchers but don't have your DVC card with you when you (try to) exchange them, but I think it involves flogging.
We went ahead and purchased ours through the DVC website (it links you over to the disney ticket site with the discount). We got our AP voucher green cards in the mail in about a week (it cost $3.00 for shipping). We figured buying in advance meant we could lock in our price in case of any price increases. We'll activate them on our next trip at one of the parks (we have to show our blue DVC membership card and driver's licenses). We were able to use our Disney Visa card reward dollars toward the purchase online and then we paid the rest of the AP cost with a regular credit card - nice.