Going along with what Hot Lunch has stated above. At my school, students with less than 3-4 years of training wear the economy lightweight cotton uniform issued by the school (Sun brand, Choi Brothers Int. out of Chicago, IL fwiw). After a few years, many opt for the school issued heavy weight uniform (Also Sun Brand, both 10 oz and 12 oz). Some opt for higher quality uniforms, some custom tailored, others not (Shureido, Tokaido, Hirota, etc.)
Beyond breathability and durability, the biggest difference I see between lightweight and heavyweight uniforms is the "feel" of them. As noted by the pictures below, the lightweight uniform hangs on you, whereas the heavyweight uniform supports itself a bit. It both looks different to the observer (possibly important in forms/kata competition) and the wearer (more important to me, personally).
I also prefer to have a "Sunday Best" as Hot Lunch stated above. It is merely a personal preference, but I prefer to represent my school, teacher and organization the best that I can; including my presentation. If I'm merely sitting at an examiners/judges table, I personally don't want to be wearing something yellowed and crusty.
Personally, I view it similar to any other professional setting. I wouldn't expect a coach in any other sport, or a senior in any professional setting to be wearing an old workout shirt, team jersey, or dress shirt with stains or holes in them.
I wash my uniforms after each training session and air dry them. After a year or two, all of them begin to yellow around the arm pits and collar for me. I try to keep one uniform that is "pristine" for formal occasions.
I think it's about personal preference and maybe even individual school norms. Some folks like to show you the years of training by wearing the same old uniform and belt they've had for 10, 20, 30+ years. Some prefer to purchase new ones, when their old ones wear down.
For day to day training, my personal preference based on comfort, reaction to movement, durability, and presentation are the heavy weight uniforms. They tend to last me 5 years or more, as long as I have at least two uniforms in my weekly rotation. I will admit, I trained for 30+ years before I purchased "premium" uniform. Prior to that I predominately used heavy cotton or polycotton Sun Brand uniforms. I can honestly say I see a significant difference in the uniform design, construction, and feel between a $60-80 heavy uniform vs a $250-350 heavy uniform. It's a bigger investment, but one I am personally willing to pay.
So to summarize:
Lightweight uniforms are:
-cheaper
-breath the best
-less durable
Heavyweight uniforms are:
-vary from marginally more expensive than lightweight uniforms to significantly more expensive
-breathability varies depending on fabric composition, fabric weave, engineered vents, etc.
- tend to be more durable for grabbing, pulling, etc.
