Budget - you can spend anything from about $100 to MUCH more.
Looks - you're more likely to use something you love! It's OK to choose the one you love the look of...and try it on for comfort.
Shoulder straps padding - this differs quite a bit so find out what you like.
Adjustability - how easy is it to get the straps right for you? Does it go big or small enough for you?
Cinch-ability - can you adjust the width of the seat to get the panel the right size for your child?
Leg padding - only some carriers have legs out padding. Is it a feature you need or does it not make a difference to you?
Fabric - Onbuhimo come in lots of fabrics like jacquard woven wrap fabric, cotton, linen etc.