If the price is the same between American and Expedia - I would buy it directly from American. It is just "easier" to get "plane vouchers" for the difference if the price goes down.
Technically you can also get them if you go through Expedia also. If you go through Expedia - you will need to go directly to AA to get your refund. Expedia will not issue vouchers.
If you book electronic tickets - you can do this over the phone. They will mail you the voucher. If you book paper tickets - you must go to a ticket counter to get them exchanged.
Your refund is "a travel voucher" good for one year. It is not cash.
Just FYI - I have had much better luck getting vouchers when the price decreases when I book directly with the airline. Expedia will not help you and I have had the run around through the airline. This is not to say you can't do it - but it can be more difficult or piece of cake depending on who you are dealing with on the phone.
Next, when you check prices - you can use websites (Expedia, Orbitz, airline, etc. . ) but note that these prices vary. I have only had luck when the "airline" says that the price goes down, not when Orbitz lists a cheaper price.
Lastly, I have most of the time booked over the phone or at ticket counter. I have often noticed AA's website showing lower fares. If their website shows lower fares but if you call and they quote you higher fares - they often say that "those fares are web fares only and we can't refund you." I have never tried to purchase a web fare and had the web fare go down and then try to get a voucher. I asked an AA rep once how it worked that way and they couldn't tell me.
Lots of iffffs . .. .
Saying this - I booked over the phone a month ago for Vancouver at $575. The price went down to $490 and I have a voucher in the mail to me.
Hope this helps some. I know every situation is different and every airline handles things a little different.