I have personally had my Ergo carrier since 2006 and I still love it.
I like that the Ergo straps more padded/higher density foam.
Beco has an insert so you can easily slide a child from front to back even at a young age (though still a little scary because hands and feet can get caught up in the wrong place.)
Ergo you put on the waist pick up your child, put the shoulder on , buckle the should strap and you are done.
Butterfly 2 you put the waist strap on, you put the shoulders on, buckle the shoulder strap, then put the child in, then buckle the cloth to the shoulder straps. This is a pro and a con in my opinion. You really have to direct the child's legs into the carrier (like putting them into a jumperoo, which as any parent knows can be a real pain in the butt when they are fighting you because they are tired and want to sleep). The problem being you could never take a sleeping baby out of the carrier without waking them (because you have to work the legs out), which to me is a HUGE flaw. The pro of corse being that if they have good head/body control you can use 2 hands to get them out of the carrier or undo other buckles (which is good cause you are going to need it.)
I do love that ergo has the pocket. I think Beco should have left a slit between the pretty print fabric on front and the solid color so you could shove a diaper or something in there.
For my daughters size (8 months) I really liked the hood on the Butterfly II. It was higher quality than the Ergo carrier which just seems like a sloppy piece of material. However the hood on the Beco does not fit very well with a 3 year old's head.
Beco does have nicer fabrics and people usually say I like how they are not a big corporation, but really that doesn't exactly work anymore (since they are now just as big as Ergo) and they both were started by a mother making a carrier for her baby and they are both still ran by the mothers who created them.
All of that said, my brother choose the Beco Butterfly 2. He felt like it was a safer carrier because of the extra butterfly insert. It also fit his waist without buying an extension piece.
If I had no carrier and had to buy again I think I would just go with whatever I could get the better deal on, which means I would probably end up with an Ergo again.