I went for the 13″ last week in replacement of my old 15″. Quite happy with it, heating can be troublesome though (but not as worse as my previous MBP). size/weight ratio is ok IMO, SD card is a plus for me because I bought some HD camcorder as well. Only drawback is the use of the stupid new display port (220HKD per cable for VGA or DVI). The boot time has also increased a bit.
Also the 13″ doesn’t have a mic input or dedicated speakers on the side of the keyboard.