Paux

Thats a very long conversation..  There are many posts on the forum where I have made it.

But briefly..  

Thorium, Lanthanum and Cerium for the purposes of DCEN manual GTAW are a push.  A welder won't tell the difference on regular DCEN Tig work..  All 3 have great arc starting, great current carrying capacity and longevity under normal manual operations.

For AC operations... Thorium has a tendency to degrade in bad ways when it is overheated... Cerium and Lanthanum have the same approximate current carrying capacity for AC GTAW work.. But Cerium and Lanthanum hold up better at the higher amperages..  When they do begin to overheat they do not split, split or grow spikes in the same way thorium does.

Even with transformer/rectifier AC welding power supplies (not just inverters)... Lanthanum and Cerium electrodes can be prepared with a pointed tip, which produces a more favorable arc plasma column which in turn produces a superior penetration profile for both fillet and groove welds compared to a balled end that has been traditionally used for many years.

So while for DCEN operations Thorium is equal to Lanthanum and Cerium  it is not superior.   For AC operations  Lanthanum and Cerium are superior to Thorium.

I would choose Cerium and Lanthanum as the more versatile electrode for all applications.  The price difference between Thorium and Cerium and Lanthanum is so small as to not really come into the argument.

Plus... If you use Cerium or Lanthanum.. You need no alternate electrode for AC operations... You can sharpen both ends of your electrode and not have to worry about identification as the Cerium or Lanthanum can be used for both AC and DC.

Hope this helps more than my first (less than helpful post).