First all these sizes for Gutterman and Amann threads are in metric ticket numbers, bigger nubmers are thinner threads. (it's, roughly, the number of 1km hanks of thread you could make from a kilogram of thread. Tex is weight in grams of a kilometer of thread; roughly, there are allowed ranges.)
Tera is spun continuous filament polyester thread. That means it's endless strands, spun into thead. Mara is staple polyester (meaning relatively short pieces) spun into strands, and then two of those spun together. The stuff sold as 'sew all' is just mara 100, in smaller spools; it's not impossible the lubrication is different, but that doesn't matter to anyone not sewing at high speed. (And if you have domestic machine, you're not.)
Rasant is continuous strand polyester, that has a covering of staple cotton. For a given linear density, a plain continuous filament thread like tera or A&E's Anefil will be stronger than a covered one. (I'm pretty sure Amann make one too, but I'm not familar with their range of products.) The cotton covering gives it better needle heat resistance, which doesn't matter if you're not sewing at sustained high speed. It also gives a slightly better disappearing thread in seams. I don't know that any of that matters for most of the things people here are going to use it for.
I suspect Roger's preference are based on good experience. Amann make fine thread, but nothing better than any other high quality industrial thread companies do, and nothing worth buying from australia if you're in the US. Some of the things he says about other threads, which are wrong, make the cynic in me wonder if he's a shareholder. There are any number of companies making thread that are perfectly suitable for making lightweight gear. Selection should be based on quality, ease of sourcing, and price, pretty much in that order. If you live in or near a garment district, you should be able to get all sorts of quality thread.
About the 'Water resistant' coatings: if you want water proof seams, seal them. Coatings won't hurt, but the thread still wicks water in.