My DS is almost 23 lbs (between 20-23) and honestly I like the padded rails with him-- I will like it even better in the fall-- it is just too hot right now for any type of sling. BUT I tried to wear my dreambird today-- in the house in the A/C and DS cuddled right in! I hated to take it off but he wanted down after a couple of minutes :(
The shoulder padding does help-- it feels like it isn't there because there isn't any bulk to it, KWIM? Yet makes the sling incredibly comfy!
Having both types of slings may be a better choice-- you can pack a unpadded sling easier than a padded but IMO if I were going to wear DS for any length of time in a sling I would reach for my dreambird. It is totally dreamy!! HTH