Any advice on a comfy, easy-to-use carrier for long walks with a 20+ pound baby -- particularly for back carries?  I'm wondering about the Ergo, Patapum, or Sutemi, or maybe the Baby Trekker or something else that I don't know about yet?  So far, I've mostly been using a pouch or mei tai/ABC and sometimes a ring slip or wrap, but as he gets bigger, I'd like something a little more comfy for long walks and ideally easier to put on than a mei tai.  Any advice?  Also, on the Patapum, what about the regular vs. toddler version?  Thanks for your help!