Hi, HRJ! Seven years ago I would have been at LLL in Kensington MD -- does that ring a bell? I'm in central NJ now and used to go to LLL meetings in Rocky Hill.
In terms of your best bet for a structured carrier for a toddler, I have used and loved both Bobas and Kinderpacks. I have a super-tall, super-heavy 4 year old and she still fits quite well into the Kinderpack (it's a toddler size, so it's much bigger than most SSCs). The Boba is a really well-designed carrier too, and definitely taller in the body than the Ergo, although it's not as wide at the base as the Kinderpack. I think the footrests help a lot with baby's leg comfort and leg positioning to make up for that, though.
I haven't ever gotten my hands on a Bamberoo, and when I tried an O and A I didn't care for it much -- especially the lack of a chest clip. I find that most moms need a chest clip for a back carry to keep the straps from slipping on their shoulders. Of course, these things are really a question of personal preference, body size and frame build, and baby's size! Carriers are a lot like shoes or jeans -- what fits one person will not necessarily fit another.
If you haven't already, you might want to check out the SSC forum at The Babywearer: there are a ton of threads on these carriers and you can search pretty easily.
You can also check out the FSOT forums there as well as here to see if you can get a good deal on a gently used one so that you're not buying new on some of these, especially the harder to find ones.
More generally speaking, I see it a lot that with the more choices we have, the more we drive ourselves crazy trying to find the "best" or "perfect" carrier. Although the thrill of the hunt can be a lot of fun, it can also drive a mama mad! When my first was little and the original Ergos came out, it was such a vast improvement over what was then available it was like a revolution -- and many people got and loved Ergos, myself included, because I had been wearing my son in a big bulky frame backpack before that. These days there have been so many variations and new brands that it's almost paralyzing -- but if the Ergo was the only choice compared to say a Snugli or Baby Bjorn you'd probably be perfectly happy, and getting one of the four carriers you mention above, you'll probably be pretty happy too because all are good options generally speaking. So this is all a very long way of saying -- don't let the imaginary "perfect" be the enemy of the good. The point is to be comfortable, get your baby on your back, and get out and enjoy life with your hands free!