I've been wanting to get RDR forever now but I dont know if I need the game of the year edition or if the normal RDR game will suffice. Thoughts? 

Hmm, I got the game of the year edition, as it wasn't much more. But have only played the regular version so far. You want to play that one first - and it alludes to the what happens in Nightmare. I started it, but it's a bit of a spoiler.

If you get the GOTY edition, it does give you access to a better horse, which is only in the DLC. Also, it depends how much you like Zombies. There is plenty of game to play in the regular Red Dead Redemption.

I'm curious about how others answer too!

u can play undead nightmare without rdr if u bought the standalone retail copy. Im not sure if it is the same situation if you download it, you might need rdr. hope this helps

edit: read the question incorrectly. go for the goty. Undead nightmare is amazing

You can get the GOTY edition cheap