The LA series engines are bushing bearing, with an air bleed carb. They are relatively light for thier displacement. Expected performance is good for trainers and slower sport aircraft.

The FX series are ball bearing engines with a dual needle carb. They are much heavier than the LA, have a longer life expectancy, and will produce considerably more useable power. (for the same displacement.)

the FX series engines cost considerably more than the LA's.

My recomendation for a beginner... get a Thunder Tiger Pro (TT .40 Pro, or TT .46 Pro) series, or a Magnum XL series engine. These are ball bearing engines, mostly compareable to the O.S. FX series, at a considerably reduced cost. These engines don't quite develop as much power as the FX... but do give better power than the LA, at alomst the same cost as the LA. Life expectancy of these engines is superior to the LA, but not quite as good as the FX.

O.S. FX engines are better for higher performance aircraft... They will survive running at higher rpm than the TT or Magnum. But, it will be a while before you would want or need the extra performance, and you may wear out your first engine before then.