Stem mount is exactly that, they mount on the stem (the part that holds your handle bars to the tube coming out of your fork. this is just another way to mount aero bars. If someone tells you it's more anything they are trying to sell you something. It is also considered "flat out, down right stupid" to try to clamp anything on a carbon tube, so if you have carbon bars you will need a stem mount aero bar. Other than that, just make sure that what ever style you get that it fits on the diameter of your stem or bars (be careful as not all are round and not all are a constant diameter. I bought a pair of clip on bars that are just a little too small for my handle bars so they won't mount where I want them, which gives me a very wide aero position.

Now, my $0.02. their are two types of bars, "S bend" and "ski jump" bars. The S bends look very flat, while ski jump bars have a sharp almost 'L' look to them (do a google image search). There is a thread from a while back that has a link to an article on these two types of bars and how they are different. The article is awesome, you should find it. In the end it depends on what type of bar you are more comfortable on and which type will allow you to get the rides done that you want to ride. Beyond that it's materials of construction and who made it. Unless you have a silly light bike don't worry about carbon, just go for Al. Try to find a bike with each type of bar that you can demo (if only in a trainer) so see what you think. enjoy, hope that helped.

If you are looking at the "Profile Design Lightning Stryke Lava/Hammer Stem Mount Aero Bar", the bars only mount to their stems. Ergo, you'd also have to buy aero bar + new stem. While stem length, rise, and bar diameter are standardized, the width and shape of the "tube" portion of the stem is not standard. As such, the stem-mount aero bars usually require matching stem components. Finally, IMHO, I would never mount an aero bar (or any bar) to a "carbon" stem. Stick with an aluminum stem. The pressure those stem bolts put on stems (which are so easy to over-torque and crack and corrode), and with the shock and weight stems support, I'm not too worried about the 28g/1oz difference between an aluminum and carbon stem. Look at your stem. It has two little bolts that afix it around the fork tube (and a top compression cap which doesn't do more than maintain pressure on the star nut inside the fork's headtube and compress the spacers and headset a bit). A hairline crack in either bolt hole and your entire upper body is going face first into the front wheel. Also, in sprint and climbs, I don't want any flexion in my stem. (Sorry, didn't mean to threadjack your post into a rant about carbon stems.)