I get all my Merino on Steepandcheap - I refuse to pay the inflated prices everyone has and wait for S&C to have a good deal. I've bought several brands on S&C and here's my take by brand:
Stoic: Fit is very athletic (read: get a size bigger than you normally would) and the colors are kind of garish and the material is thin and rougher than the other brands. I haven't had any issues with it falling apart, but probably wouldn't buy Stoic again as I'm just not really impressed with their Merino shirts.
Smartwool: Excellent cut, nice soft material, good fit, but has a tendency to get holes in it. I'd buy it again, but this one requires more babying (handwashing, line drying, care when wearing) than any of the other brands I've tried.
Icebreaker: Athletic fit, excellent material. No complaints at all. Love Icebreaker, it's my favorite brand of them all.
Sport Science: My heaviest Merino sweater and hat come from Sport Science. It's not the most attractive (Olive drab green) but it wears like iron and is an excellent material. I would buy this brand again.
Luna: Very similar to Smartwool. Nice flattering cut, soft material but has a tendency to get holes. I'd buy it again, but you really have to baby it.