On PSN, there's a "RDR:Undead Nightmare" which is 9.7 gigs and is $10.49, and there's a "RDR: Undead Nightmare Pack" which is add-on content and is $4.89.  What's better about the bigger one? I can't see any difference in the description.

Actually, I'm not sure. I'll go check.

I'm not entirely sure, but I believe the bigger one allows you to play it without the original game. The addon one requires that you own Red Dead Redemption.

Damn. I saw the $10 one first and immediately downloaded it, not seeing the other one until later. 
 
Whatever, nothing I can do. At least now I don't need to swap out the discs?

I bought the bundle pack that has both RDR and UN and it's the standalone version. It sucks in my case since I could have saved the $5 and gotten the DLC bundle instead (which I got too. I don't need 18 gigs taken up so I can play this game. 10 gigs is all that is neccesary.