I haven't chosen yet but make sure the one you get doesn't allow their legs to hang down. you want it to support their hips nicely and allow their legs to start bending at the knee
The boba and ergo are the best ones there. They allow the weight to be better distributed. Most sits on your hips. I have an ergo. It's supposed to be better for the baby too won't dangle them in the crotch area but instead provides a comfortable seating position. The bjorn will kill your back.