From what I understand a 4 ohm speaker will be more accurate than a 2 ohm .
You have enough power for either with that amp.
I would get the 4 ohm because of that and because it will allow more adjustment for the gain . With the stock speakers (the 2 ohms) There is no adjustment range for the doors at least they have to be set at minimum gain.
http://www.e90post.com/forums/showthread.php?t=368847
If you go through Musicar they will give you the right stuff.
All the right stuff.