I am not sure your specific question on fabrics (and I'm probably not qualified to answer specific questions), but 40D, 70D, 75D refers to the thickness of the fabric. The bigger the number the thicker it is (and presumably more durable and heavier). 1500mm, 1800mm refers to the water proofing the fabric has. The higher the number the higher the pressure of water it is able to withstand before leaking. As far as I can tell, those numbers are mostly marketing gibberish. Water doesn't get applied to the fabric with pressure. The question is how well did they waterproof the tent? Are there any tiny holes they missed? Does the geometry allow water to pool? or drip on you when you get in or out? Marketing won't tell you the answer to those questions. Experience and reviews from experienced people are really the only thing available. No experienced person I know use either of those brand tents (and they are heavy!).

OM recommended some good "cottage" tents. I understand they are out of your price range. If you can find used, that is a good idea. I just bought a tarptent and it is a wonderful tent. My entire family of four slept comfortably in a 4 pound tent.

Trying to optimize quality and cost I typically look at Kelty. The trail ridge 2 is in your price range. The TN2 looks like a pretty nice tent if you can find it on sale. Personally, I wouldn't want to share a 2-person tent with another guy. A tarp would probably be less confining option. I share a tent with my family. When I go out with friends, we each bring our own shelter.