I have the Beco Butterfly for my 25 lb 8 mo old. I can wear it for hours with no issues, I love it!!! I have heard great things about the Ergo as well, I just found the Beco more attractive.

You also might look into the Sutemi. DH wears one for my ODS (3.5 yrs old, 38 lbs). He does a back carry with no problems, it can also be used as a front carry.

Stacy