I'd say the more resolution you can get, the better. I shot a "life size" (about 64 inches tall) poster of a model for an energy drink campaign and used a Canon 1DS MKII (16mp) and felt the resolution wasn't enough, even printed at 180ppi. There was a little cropping that had to be done to the original file to fit the format of the poster and lots of compositing, thus compromised some of the file's integrity in the end.
The D3X might be enough, but if you can get the H3D, I'd go with that assuming you're comfortable shooting both cameras. If this is a commercial job, you'll probably have to leave room for type or to provide cropping options for the art director. Depending on the mock ups the AD has (if any), you should probably have enough lights to create a smooth/even tone background in case they want to crop the model out of the background all together. I'd also shoot with a longer lens like the 85mm for Nikon or the 120mm for Hasselblad with the camera on a secured tripod with a camera height near the model's belly button. That would help if you need to composite multiple parts of the frame together to make several so-so shots into one good shot.