I think it important to note that "rubber quality" was 4th or 5th in the notes above.
Without talking rubbereze.. (I don't speak that language) ...
If you are looking at a sports store (not a dive store) than you can bet that the inexpensive 3mil wetsuit that you see is not made from a quality rubber and may be fine for surface sports, but will not provide much insulation under pressure due to compression. Much has to do with the consistency of the bubble size and distrubution within the "rubber" and how those bubbles get there...
At a dive store, you can start with assuming that feel will tell you a great deal. If the suit is easily streatched, (GREAT for getting good fit, flexibility in the water, comfort and for easy entry and exit - don't underestimate how hard it can be to get a 7mil suit on and off... ) you can expect that it will compress more under pressure so have slightly less insulation... but "warmth" is not the only feature of importance - diving is much less enjoyable in an uncomfortable suit - and I have seem some folks huffing and puffing to get their suit on or off as a result of the material of the suit. Try a bunch on. Different manufactures have slightly different cuts and you will likely find one or two designs that fit you much better than others - then, if available in that line, select the quality of neoprene based on flexibility, function and price that you desire.
My $0.02
Have fun with the research, purchase and most of all the diving!