12GB of RAM is probably more important than either the 950 or the SSD
I personally wouldn`t take 12GB.
I`m running with 8GB and I`m 5boxing. Even in WG and dalaran my system never uses more then 5.5GB. I tryed with 4GB at first but it was just horrible in every new zone.

Or just take the 12GB for future 10boxing maybe :-)