First up I'll say I don't have either of them so I'm not biased either way. The difference is that they're designed for different purposes and each has their own advantages. The LA is a beginner or sport engine where the FX is more of a high performance engine. Generally this means that the LA will swing a big prop about as well as the FX but for speed freaks the FX is the choice.

The FX crankshaft needs ball races because it has a much larger gas passage which means a much larger diameter crankshaft. Combine this with the higher revs it needs to get max HP and the surface speed of the crankshaft is too high for a plain bearing. At the lower surface speed of the LA crankshaft the plain bearing is more than adequate and has the added benefit that there's nothing to rust and will last a lifetime given the correct lubrication. Also a plain bearing doesn't noticeably rob power at the revs it's intended to run at otherwise the front of the crankcase would get hot from friction, but it doesn't.

The LA carb is simpler than the FX and while it may not be as precise it's more than adequate for the job and easier to adjust.

Just because both engines are OS is no reason to slag the one that's got less power, they're designed for different purposes. I could ask why anyone would even consider buying a wimpy FX when there are engines like Rossi, Irvine, Webra, MVVS, Jett and Nelson out there