Mel,
the patapum looks similar, except that it separates the carrier between babies (5 months to 40lbs) with toddlers (above 18 months), whereas Ergo is good from 4/5 months to 4 years (40 Kgs)
I think tho, since 5 months to 4 years is a long period, a person may need more than 1 ergo hahaha.. cos it's a wear and tear thing, right? so the patapum can be an option.. I took a look at the webbie and it looks good, except for back carrying, for Ergo you dont need assistance, for patapum you need.. but maybe for Ergo it's cause the model is expert in shuffling over her baby kekekeke..