Lose weight, conserve power.
The ARX LTD Stem is the only stem in 3T's road line made from carbon fiber. While we're not sure why the Team stem didn't get the same treatment, the end result is that the ARX LTD drops an additional 7g of claimed weight compared to the closest alloy competitor in the Italian manufacturer's component armada. The LTD model's carbon construction also means that it's the stiffest stem in 3T's line, so you'll be that much more efficient when you're hammering out of the saddle and every watt lost to a flexing front end is one less you have to cover moves or launch your own attack.
Compared to the improved power transfer, the stem's minimal weight savings are, well, minimal; however, when you've already trimmed the maximum grams from your frame, wheels, and drivetrain, dropping minimal weight in the cockpit may be what pushes your non-UCI-legal ride under the coveted 14lb mark.
If your quest to drop weight has also led you to a carbon handlebar, then rest assured that the wide spacing of the ARX LTD's faceplate clamp splays the bolts out nice and wide. This diffuses the clamping force across a wider area so that less torque is needed for a secure fit and the torque you do wrench into the titanium bolts isn't concentrated on a narrow spot on the bar. The light texturing inside the clamping area also helps to achieve a firm fit with less torque, and it reduces the risk of carbon-on-carbon creaks and pops when you're using carbon bars.