Is there some bonus to buying them online rather than the store?
You can pay with Target GCs and use your Target REDCard as the backup method of payment. If done the right way during checkout, you get a 5% discount off of the face value of the Disney GC.
FYI, I bought a Disney giftcard in store and got the 5% off with my redcard. I wasn't using any GCs though, just paid with redcard. So if you aren't using any GCs or other method of payment it sounds like it's worth it to just buy in store and pay with the RedCard