I called The Rain Shed, as they've been very helpful in the past, and spoke their buyer, who said I'd be welcome to test a few samples. I hadn't been there in a few years (used to live close by, but no longer) so I drove down to Albany.

She let me tear fabric to my heart's content, which I much appreciated. She also passed along some helpful information, based on her experience. Understand that I'm not quoting her directly, and I don't want to put words in her mouth. This is just my understanding.

Apparently, in years gone by, any fabric labeled, say, "1.9oz coated ripstop" was likely to be pretty much the same. These days, manufacturers use different weaves, materials, processes, etc. in order to cut costs and appeal to different markets. At least some "ripstop" weaves are mostly cosmetic.

Also, fabric normally tears more easily in one direction than the other. I guess I didn't realize that.

I tear-tested these while I was there:

400x300 diamond ripstop - looks identical to what I purchased elsewhere, and tore in the same way, that is, easily in the longitudinal way (warp, I think) and very difficult in the other direction (weft).

1.3oz (30d) silnylon - very difficult to tear in either direction. This is "silicone impregnated nylon." There's also silicone coated nylon (coated one side), and nylon coated on both sides with a combination of silicone and polyurethane. Both of these tore pretty readily, and were difficult to tell apart visually from the impregnated nylon without looking very closely.

70d silnylon (should be about 1.9oz, but not specified.)- tore pretty easily in both directions. I think this is coated, not impregnated.

70d (1.9oz) PU coated ripstop - tore fairly easily, maybe a little tougher than the stuff I had ordered before, but not much. As before, I couldn't really detect much of a rip-stopping effect.

210d coated oxford - tearable, but of course tougher than the lighter material. Quite a bit tougher than some of the same denier and weight that I used to have. That stuff was so weak I never used it, and assumed it was to be avoided. Maybe not.
I have a piece of 210d ripstop (that I got from RCT Fabrics before they quit business) and it's very hard to tear. I mentioned this to the lady at the Rain Shed (stupidly didn't get her name) and she said it was probably a special order made for some manufacturer.

So, I guess what I learned is that it's kind of a crapshoot, and I should worry less about it. Still, it irritates me that you can buy "ripstop" material that isn't really ripstop.

I bought some of the 210d coated oxford, and in case anyone's interested, it's 2.95 oz/sq.yd. The 400x300 diamond ripstop is 4.56 oz/sq.yd. Measured in a highly scientific manner with a tape measure and a postal scale.

Interesting that the 210d oxford weighs only slightly more than the 70d 1.9oz PU coated ripstop (2.7oz). Maybe the coating on the 70d is that much heavier.

Also I notice that some web sites (like the Rain Shed) are listing fabric weight by the linear, or running yard. I wish they wouldn't do that. It frightens and confuses me.