A while back, someone on this board posted a forceful argument that Thai Nakorn was better than Renu Nakorn. (I can't find the person's message any more.) This made me resolve to try the two back-to-back. I've now done this, and though my methodology was not as scientific as I would like (one visit to each, ordered different dishes), the result was clear: Renu Nakorn.
At Renu Nakorn I had chicken larp and kang hung-lay (pork stew from the Northern Thai section of the menu). Unlike the sausage salad that I had there once before, the chicken larp did not blast me clean out of the solar system. But it was good: complex and filled with clean, distinct flavors, probably the best larp I've had. It went down real easy. The pork stew was located in taste-world somewhere between a vindaloo and a Thai peanut sauce, which is understandable given the part about Northern Thailand. It was a very satisfying dish, warm and comforting. It appeared within minutes of ordering so I assume that they keep some warming on the stove, but if so then it must have enjoyed simmering. It was darn good.
At Thai Nakorn I had larp pladuk (catfish) and "boar with spicy sauce" (from the specials at the front of the menu). Both had virtues, but both had significant problems. The larp was dry, and it required frequent hits of water to choke it down. The tastes weren't clear or distinct either. Although it clearly wasn't chicken or pork, you couldn't have guessed it was catfish. The boar with spicy sauce was interesting and complex in its flavors, with a good acidic edge, almost citrus, cutting against the chilis and meat. But the meat was leathery. I realize that a boar is a leathery sort of animal, and the cooking did remind me of some Szechuan dishes where the meat is deliberately rendered somewhat crunchy, so conceivably it was supposed to be like that, but in any case I had a more pleasant experience eating the pork stew at Renu.
At both places I ordered the food "very spicy", and I made emphatic-looking gestures in a lame attempt to ensure that I was getting the real stuff and not being treated like a tourist. I do have to say that the larp at Renu Nakorn and the boar at Thai Nakorn fit the description of "very spicy", as measured for example by the amount of time I had to spend blowing my nose while eating them. Why I would want to spend much of my meal blowing my nose is a topic for a different forum. Suffice it to say here that those dishes were darn spicy. Some people claim that excessive spiciness gets in the way of the flavor (which is stronger than just saying that spiciness is being used to cover up for a lack of flavor), and I spent a bunch of time reflecting on this while I was blowing my nose. My own feeling, just for myself, is that the extreme spiciness does make me work harder to identify the flavors, but I am not persuaded that it prevents me from finding them.
The person whose polemic in favor of Thai Nakorn and against Renu Nakorn originally motivated me to perform this test also forcefully asserted that the Renu Nakorn people are insecure jerks, and that this helped explain why the Thais all eat at Thai Nakorn. While this assertion strictly speaking has nothing to do with chow, I have to say that it caused me to do a lot of empty philosophizing: if people are jerks, can they truly make good food? Are they merely tricking you into thinking it is good? That sort of thing. Having gotten off on the topic, I did some investigating, e.g., by talking to the people, requesting things that were out of the ordinary, etc. My impression, I have to say, is quite the opposite of the polemic I read here: the people at Renu Nakorn, in my experience, were much nicer and more easy-going. This was a relief, actually, since as mentioned above it's my view that they make the better food. Although this particular meal at Renu Nakorn wasn't quite the revelation of my first, I could find absolutely nothing to complain about, which is unusual in itself, and all the flavors were so clean and fresh and fascinating that clearly a lot of work had gone into them.
Then there's the question of who was eating where. It's true: in my two visits to Renu Nakorn I haven't seen any obviously Thai people eating there, while maybe half the people at Thai Nakorn were Thai. On the other hand, the people at Renu Nakorn were a very pleasing tableau of Los Angeles, with numerous ethnic groups represented, which was not true at Thai Nakorn. Obviously this isn't enough data to draw conclusions about the Thai community's preferences between the two (I mean, c'mon, maybe they just live closer to Thai Nakorn). But I admit that it remains an interesting question.
That's my report.