I have a Maya Wrap ring sling. The padding is nice - especially when you're carrying for a long time. I also think it helps with positioning for newcomers. I have one without padding, and it can get a little "diggy." I also measured for the medium, but went with the large for nursing purposes. I like having the extra length, but that is totally a personal preference. I can't speak for the quality of the other brands you mentioned, but the Maya is excellent quality. I definitely recommend it.
I have a ring sling from Sleeping Baby Productions (it's the linen one). I actually tried out one like this and the Maya wrap at our local babywearing group and chose this one because I liked the shoulder better. The one on mine is unpadded and pleated, so you can spread it out so that it goes over your shoulder and down your arm a bit. The Maya wrap shoulder is padded, but you can't spread it out. I thought unpadded and spreadable was more comfy (and so did a lot of the other moms in the group, who told me to spread it in the first place!), but your mileage may vary. The quality of my sling is excellent and the "babywearing expert" who leads the group (she does lessons on how to use pretty much every carrier ever made) recommended the lady who runs Sleeping Baby Productions as being extremely safety- and quality-conscious. (My sling is also MUCH cheaper than the Maya Wraps...mine cost ~$45 and there are cheaper ones available.)
I bought a medium based on my height (I'm 5'8"), but the tail is actually really long, I think because my shoulders are pretty narrow. I tuck it into the sling so that it doesn't get caught on stuff or trip me. I would not recommend deliberately getting a longer sling for nursing wrap purposes just because the tail down to your knees is a little bit scary, honestly.
Oh! Almost forgot. The lady at Sleeping Baby Productions does shoulder conversions on slings for about $20, so if you get the Maya Wrap and don't like the shoulder, she can change it for you.
(I have no connection to Sleeping Baby Productions, I just love my sling! :) )
Mom to the wacky and wonderful Kalyani (August 2011)
To answer your questions:
1) No. I find it easy to adjust.
2) No experiece with those.
3) I am a medium size but wish I had ordered the large, as the tail is just a little too short to fully cover. It does have decent coverage though, but large would be much better for me.