I actually just happen to have recently received the sports pullover short binder that I ordered, so I can offer a bit of insight for you.
I don't see why ordering a size smaller would be of benefit with that one. When I first pulled it out it looked really small, and I was nervous about it fitting at all (I measured to a 3XL, and ordered that size), but I gave it a shot all the same. It actually stretches as needed, but somehow it still compresses remarkably well given that fact. Not to mention: it is comfortable -- man is it comfortable -- and I really hardly noticed it at all. I think it's comparable to the high performance short binder that I've had for about eight months now (give or take), but I don't need to worry about the velcro, it's easier to adjust with, and it's more comfortable. Because it's not as restrictive, it's really easy to breath in as well.
So, from my experience, you should be good ordering the medium, though if you wanted to go a size smaller you could probably do that as well.
As for the v-neck, I can't say anything to that one: no experience.