I wear a 12 oz and a 14 oz. I'm looking for a well made 10 oz. that fits me right and doesn't break the bank.
I don't think the weight itself is what makes heavyweight gis better; it's the material. The lighter weight gis are typically a twill material, kind of like jeans, only a lot thinner. Heavier gis are typically cotton canvas, which if weight was kept equal, would be more durable and stiffer (in a good way). Basically the difference between a 100% cotton t-shirt and a 100% cotton dress shirt.
I've been looking into the higher end poly/cotton blended gis, such as Shureido's New Wave line. I don't know anyone who owns one, nor have I seen one in person. I hate that this stuff is all mail order. I'd love to be able to go to a place like Sports Authority and try on a bunch of different high end gis.
And with gis, don't be afraid to spend a good bit of money on them. I've owned a Shureido K-10 which is about $225. Easily the best gi I've ever worn and wrath the money. I wore it 4-5 times per week, for 2-3 hours at a time. 6 years of that, with bleaching and machine drying it every time (both a huge no-no for gis), and it was pretty much just as good as when I bought it. I'd still have it, but I lost it in a move from grad school. Some aren't worth the money, but the top brands like Shureido and Tokaido (their Japanese made ones) are. I'd easily justify the $225 of the Shureido over the $65 for my KI heavyweight. I'm wired like that though; I'd rather spend $200 on something I'll get a lot of use out of and enjoy than spend $10 on an equivalent product that sucks and I'll throw away.