Hey nhilbourn,
Thank yo ufor your comment and it's really good one and confusing as well
I would first like to thank trutherous for really great answer!
As he had mentioned, ~데 could mean in a various way depending on what comes in the front.
It could be used as and, but, or even the ending when you are reluctant to say something.
On the other hand, ~대 shows more of the second hand. When you have heard something or so, for example 'Amy said she would go for shopping' then you could say '에이미 시장보러간대.'.
So the sound is almost the same but the usage or the meaning could vary a lot depending on how it gets combined with other sentences and subjectives or so.
Does that make sense to you a bit?:)
Thank you
Madison
Team Koreanclass101.com