I have not used any of these but here's what I can offer for the Serene SLing that you like:
-the shoulder looks narrow-like it might not spread far enough over the shoulder for a comfy long term fit. It might be OK, but it's borderline IMO
-with the knit fabric it might be great for little babies but I bet for more than 18lbs or so it will stretch too much and hang down too far for a good fit
-you CANNOT get the right fit from just the bust size. A sling should not hang below your waist with baby in it so you need to know the length of the sling/pouch. Usually pouch makers want to know your shoulder to hip measurements for a good fit. I would be concerned how to get the right size and make sure they have an easy return policy if you tried one.
The other pouch you linked looks like a basic pouch and is probably a safe bet if it's sewn well.
www.thebabywearer.com has reviews on tons of slings/pouches. You can see if any of these are listed there.