FX smooth twist barrel Vs Lothar Walther barrel. Which one is better!

Neither is better is probably the "best" answer...LOL.

I have an FX Streamline 22 with ST barrel. It's shoots the JSB pellets very accurately.

I have an AA S410f TDR with LW barrel, it also shoots very accurately but has the benefit of being able to use slugs, which is huge for me because I like to hunt.

The ST barrels are not recommended for slugs/bullets. But with the new STx barrels coming, they may offer the best of both worlds.

Can't really say one is better than the other.
I've heard LW are very accurate and the same with the ST.

I agree with you. LW barrel shoots very accurately using slugs. Maybe the slugs are too heavy, they are hard to spin perfectly with high speed in the ST barrel.

"Kversion"I agree with you. LW barrel shoots very accurately using slugs. Maybe the slugs are too heavy, they are hard to spin perfectly with high speed in the ST barrel.

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Traditional ST barrels are also choked which gives them another reason to not like slugs.

Smooth twist in both of my FX500's and could not ask for better. LW barrel on a Marauder 22 cal and it is every bit as good or better than the smooth twist. I have never cleaned any of them and they shoot 1/2 inch at 50 yards all day long. Love airguns.7

LW is better for me, ST barrells are not accurate at high speeds,

Barrels are relative to the tolerances they are made to.
Both can be just as accurate as the other. There are subtle differences.
The smooth twist idea isn't new. HK has rifles with polygonal barrels that were around for years.
The smooth twist will be less prone to fouling, due to the lack of sharp edges. (It would almost be self cleaning)
Not saying that a traditional rifled barrel can't be polished to the point that it will preform just as well.

The difference is that the way FX does the inserts is a new concept that makes it a lot easier to manufacture.
Smooth twist alone doesn't ensure accuracy. The person doing the final quality checks and the tolerances that they uphold will overall determine the accuracy.

So to answer your question?

It really depends on their final quality control. At what level of perfection does each company uphold. 

And that is the barrel that will be more accurate! (((Given that you tune the harmonics correctly to that specific barrel.))) every barrel is different.
So the end user and their understanding about harmonics and the ability to tune their gun will play a huge role in accuracy.

So IMHO. At this point because FX has a high standard of quality control, and the fact that they can tune their guns. The is where the FX gun out shine all others at this time.

Barrel quality is only one part of the equation.