The Legend and Legend LX are both fine regulators. The difference is the adjustments you are allowed to make to the second stage while diving.
On a second stage you basically have two adjustments, spring adjustment for cracking pressure (how much vacuum is required to start the air flowing) and venturi assist ( how much does the path of the air assist to keep the pressure present once the air is flowing). I am sure others can provide better descriptions of each.
On the Legend you get to control the venturi assist, the effort required to get the air to flow (cracking pressure) can not be adjusted during the dive. On the LX you can control the cracking pressure and the amount of venturi assist is tied to how this is adjusted (MBS - Master Breathing System). The vendor has decided in advance how much venturi assist you get based on how you set the cracking pressure. Atomic Aquatics allows you to adjust the cracking pressure (pretty minimal adjustments), but the amount of venturi assist is determined by your depth (AFC - Automatic Flow Control). The deeper you are, the more venturi assist you get...regardless of cracking pressure adjustments. Again, the vendor has made the decision of how much venturi assist you get.
For the majority of divers, having one adjustment is fine. Some regulators provide no user adjustments. Other regulators provide both.
I prefer some control over cracking pressure, so I would pick the LX. It is the regulator I purchased for my son and when I dive it I find it to be an excellent regulator.