I believe I have read that there's no difference in online and in-person price with WDW. (it's different with DLR, which is why the difference stands out in my head)
However, if by chance they *raise* prices between now and then, if you decided to buy at the gate it would be more.
Have you looked at the discount ticket sellers?
Undercover Tourist is a well-known one, and if you sign up with mousesavers and get their emailed newsletter, they have a special link in their monthly newsletter that gets you a slightly better price through
undercovertourist (which already is discounted from WDW). Their prices show the full price...tax is already included, and there's nothing extra for shiping. On WDW you have to go into your shopping cart to make sure you have the full price with taxes.
I personally would sign up with mousesavers, get the monthly email, get the link, follow the link to undercovertourist and buy the tickets that way, have them shipped to me and take them along.
There are a few other discounters, by the way...Maple Leaf tickets is one, I think, and there are a couple more. I don't want you to think I'm a shill for them or anything, it's just the main one that I have personally heard of.