I find the Beco more comfy, mainly because the back is much higher and it holds DD more snuggly against me. Although, in the winter, I preferred the Ergo since it was easier to get it over jackets and sweaters. My DD (26 months) prefers the Ergo since the seat is much wider (I think older babies may find the wide seat more comfy, since their legs dangle more in the Beco). The Beco folds up more easily, so it's easier to carry when you're not using it. And of course, it's prettier. The Ergo fades more easily, but the fabric is much sturdier than the Beco.
They're really both great carriers. You can't go wrong.