I've owned both. Sold my Ergo after a brief use cos preferred Beco. Still have my Beco which needs to be sold, just haven't gotten round to doing so. Anyone interested, pls PM me. It's very new. Didn't use much, only for travelling, mostly been sitting around in my cupboard.
Beco
-better for smaller sized parents. Ergo straps are wide (they will slip off narrow shoulders more easily) and bulky (big fat straps on a small frame just isn't as comfortable as on a bigger person). The padding around the hips is also much slimmer.
-much more compact. Can stuff in handbag. Ergo needs bigger bag.
-cooler. Material is cooler and cos there's less material and padding.
-looks MUCH nicer than Ergo.
-Better back support for kid as the cloth runs higher up their backs. But warmer for kid's back.
If you are small or slim, heat-sensitive, concerned about aesthetics, don't like lugging something bulky, then go for Beco. Otherwise, Ergo has more padding so more confortable when using on bigger/heavy child.