Chuckr53-RCU is pretty much right on with the differences between the OS FX and LA line. The big differences are that the FX line has a Ball Bearing (BB) supported crankshaft, while the LA line uses bushings. The carbs on the FX line are more advanced "two needle" design compared to the "air bleed" carb on the LAs.

As for why two engines with the same displacement have different power, thing about car engines. A racing 2 liter engine is a heck of a lot more powerful than what you find in a family sedan. The reasons are things like valve timing, compression ratios, fuel systems, etc etc etc.

Well, the exact same things affect model engines. The OS FX line of engines have carbs with larger thoats than the LA. The LA line, in fact, has some of the smallest carb throats you'll find on any engine in that class. A larger thoat means more air and fuel can get in to the engine, but it also means fuel draw is more critical. Smaller throat engines idle better and tolerate bad tank setups better, but produce less power.

The intake and exhaust timing is likely different between the FX and LA engines as well. I haven't timed them myself and I'm too lazy to go find reviews that give the numbers, but you can be sure that the FX line will have more "aggressive" timeing to get more fuel/air mixture in, and more exhaust out of the engine, giving more power, but at the expense of a harder to tune engine. The FX also has a higher compression than the LA, at least based on my experiences with the engines. Again, more power, but harder to tune.

So, why would OS bother to build the LA line? They are cheaper, and the OS FX line is rather expensive, that's one reason. The LA line is also designed to be easy to start, tune, and idle well. It does all those things quite well, it's one of the best behaved engines out there. The FX can be a bit tempermental at times if you are using the wrong glow plug or don't have it tuned right.

Back to the origional question, the Evolution engine is roughly equal to the OS FX line. BB supported crank, two needle carb, solid compression. In the air, the Evo and the FX are going to fly the same. Both the FX and the Evo produce a heck of a lot more power than the LA line. For that price difference, get the Evo, no question about it. If the choice was an FX or an Evo for the same price, flip a coin. For a beginner, I'd lean towards the Evo, for a 2nd plane, I'd lean towards the FX (again, assuming the same price)