Jane at QuirkyBaby is having a Memorial Day sale through today or tomorrow (I think) that includes Beco as well.
The biggest advantage to the Gemini over the Bjorn is the hip and back carry options now that your LO will be outgrowing front carries.
I've tried it and do like it a great deal for ease-of-use and comfort. (I am just under 5'8" myself; my DD is almost 15 mos and 22 lbs ... we do back-carries exclusively because I have a short torso.)
Beco is not a brand you usually find at chains like BRU or BBB; it's more of a boutique-y brand. If there's a small, independent baby-gear shop nearby, they would be more likely to carry it!
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle