Acana is AWESOME! It, along with Orijen and Go! Endurance, are probably the only kibbles I'd feed right now. Champion Foods is an awesome company.
Jackson ate Merrick's Before Grain when he was younger, I think from 5 months old-7 months old or so. He did fine on it but never really enjoyed it... he got tired of it really fast and would refuse to eat some meals, but he's just really picky. I think either are great choices but I would definitely choose Acana over BG!