If you are planning on driving this as a simple daily driver in the snow and rain, by all means go for the AWD, you will probably love it. I myself would not be caught driving a 100k machine on the beltway every day, but if you have the money to throw away go for it.
But if we are talking performance, AWD is a myth. You get a jump off the line (marginally at best), but you do not get improved handling, wet or dry. The only systems that will marginally improve handling on paper are advanced systems that will divert power to the outside wheels while dragging the inside. You will never recognize this attribute off the track, and quite frankly this is a straight line car anyway. You will never see better handling and you will not help your corners, wet or dry.AMG made an amazing engine here, but still the same weak handling.
I will say 150lbs is not a LOT of added weight, but I still attest you are being conservative.
JUST CARS YOU SAY, Blasphemy! I was not aware Gt-R was AWD, DOH! To my knowledge audi was the only one who screwed around with awd and they are proven to be smoked off the line anyway (audi helped developed c63 with amg so this is not tooooooo surprising that AWD comes as an option).
Anyway, fun thread here but as you can tell I am biased towards my bimmer (and RWD), and would love to meet up and show you how it should be off the line 
(I kid I kid).
And OP make sure to post what you went with. And pics too!
Congrats on the approval.
AWD is not an option yet for the c63. The RWD will still have okay traction as the weight shifts to the rear during acceleration, but it will walk out from you pretty well in the corners. I am just a fan of AWD and I disagree about the audi statement. An awd audi with the same hp will destroy a c63 
BMW is big on awd as well, but not for their performance lines. They are just quick in general haha
Thought I would find a video to sum up our thread. To youtube....
It's old, but pretty in line with what we have been talking about: http://www.youtube.com/watch?v=DaZExdTHHjY
The correct tires on a rwd vehicle will no doubt still be great off the line. You mention an advanced awd system that changes wheel speed in corners, well most are doing that now. I am biased towards awd as I am used to driving them and like the way they feel on the brink of taking out a guard rail, rwd on the other hand...not so much
