Hi Isvelis!
I believe it really doesn't make a difference, at least in terms of cost. The cost of the tickets at the Disney Store is the same cost as purchasing online. You may, however, want to go with the online purchase, since I'm not sure you can get all the ticket options in the store, such as Park Hopper, No Expiration, Water Parks, etc.
If you do decide to purchase online, with it being so close to your vacation you'll probably need to get the Will Call delivery. When you purchase with Will Call, you can pick up your tickets at just about any park or resort. This saves you from losing the tickets or, perhaps, forgetting to bring them when you leave for your trip...
Have fun! Yes, it will be busy, but you may be having so much fun you won't even notice :-)