Jason -
The question "What is the thinking?" implies that there ought to be some sort of informed concensus. Sadly there is not. Most owners have experience with only one manufacturer, so they have no solid basis of comparison. In other words, if I had gone through a set of Carlisles and a set of Marathons, then I might be able to offer a meaningful comparison. But since I haven't, I can only offer my impression of the one I used.
To answer some of your questions, the added load-bearing capacity of the D-rated tire does seem to make a difference. Owners (including me) who have upgraded from C to D seem to have fewer problems after upgrading than before. And the fact that TM has moved entirely to D-rated tires ought to say something.
Buying wheels (rims) that meet the expected load just makes sense. If you've read the threads, you know that there are some 1500-pound-rated wheels out there. But most wheels in the 15-inch size seem to be rated 2600 pounds, and that makes much more sense for a 4000-pound trailer.
The load rating of a tire is certainly dependent on its inflation pressure. You can (and should) go the manufacturer's web site and look up the numbers. What they seem to say is that, if a D-rated tire is inflated to only 50 psi (the C-rating), then you get the load capacity of C-rated tire, more or less. Perhaps this is not a surprise. To achieve the full D rating, they need to be inflated to their max pressure, which is often 65 psi. And yes, the higher pressure makes them harder. (I'm not sure this has much to do with a stiffer sidewall, but I could be wrong.)
Speaking only for myself, I wanted more load capacity than the C-rated tire, but I didn't need all of the capacity of a D-rated tire. I needed sort of a C-and-a-half, and I got it by putting on D-rated tires, but inflating them to only 60 psi or a bit less.
Finally, a harder tire will of course transmit more road shock to the TM body. The first lesson, of course, is to take it easy on rough roads (such as construction zones). But the second lesson is to balance those tires, so they roll smoothly, and don't generate their own shocks by pounding as they rotate. I have a feeling (but no proof) that this board's discovery of the importance of balancing wheels is about as important as going to larger tires.
So there is no stock answer that says "Brand X and Size Y is the best!" But by taking the right steps (increasing the load rating, choosing the inflation pressure you need, and balancing each wheel), you can do yourself a lot of good.
Hope this helps. I look forward to hearing wmtire's assessment of the situation, especially the issue of whether build-quality has improved.
Bill
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