ughh... I really want to get the batman bundle but I know eventually the game of the year edition that includes all of the expansions will go on sale eventually. decisions decisions...
The DLC isn't very good for Arkham City unfortunately. It is mostly just being able to play as Robin and Nightwing in the challenge rooms, and they both play to similarily to Batman to make a huge difference to gameplay. The Harley Quinn story DLC is about 2 hours total and not all that great either. You never get to move around the city as Robin so it is essentially just more challenge rooms with some story filler.
If you really want the DLC though, they have went on sale when Steam has that franchise on sale, if I remember correctly, I think I bought all the DLC for about $5.
I think You would be better off to get this and buy the DLC later.