You're a good shopper Catwmandu!
OK, so here's a picture of the Baby K'tan. I'm 5'8" and was 125 lbs with an A-cup pre-pregnancy. Based on that and reviews that said they run big, I got an XS. Right now, I'm 152 lbs and maybe a C-cup. So the fit is a little snug. I'm not super comfortable with the amount of pressure it put on K's head when I tucked her in. But I liked the snugness otherwise. I would say you could definitely wear it under clothes - it's not that bulky and you could keep one ring around your waist, but it also takes no time at all to put on. It's so easy.
I think nursing is going to vary based on how it fits you. Baby K'tan's website says you can unsling one ring if there is a snug fit. I'll try that later and see how it goes. It's definitely too snug right now for me to nurse in it. But I also wonder, as she gets old enough for other carries, if it will be easier to nurse.
The fabric is less stretchy than the Moby. Oh, and it felt really tight when I first put it on, like I would never be able to fit a baby in there. But once she was in it was just a tad too tight. I guess I just need to lose a few pounds! :) I also really like how unbulky it is.