I own both a Merino Ninja Suit and a regular synthetic Ninja Suit from Airblaster. The wool is definitely the way to go, though the synthetic is also good.
In my experience, the wool stretches a little more, breathes a little more and does not stink after multiple uses. I still wear the synthetic on colder days, but can also just put an extra layer with the wool and control my personal microclimate pretty effectively.
I snowboard in the pacific northwest. I'll wear the merino suit with just a shell from about 25F to 40F and be perfectly comfortable. Under 25F, I'll add a flannel, use an insulated jacket, or just use the synthetic Ninja Suit without an additional layer.
As with all Ninja Suits, the beauty is that snow can never get down your back and all of your base layer is organized into one piece. This means that your base layer top will not ride up and bunch with your other layers, keeping you comfortable and moving freely beneath your outer layer.
Put it this way, trying to go out without a Ninja Suit is like getting off a trampoline and trying to jump normally.