Here's a little Birkin 101.
Birkins come in sizes 25, 30, 35, 40 and 45cm. The 45 is a travel Birkin with slightly longer straps. There is also a similar bag called a Haut au Courries (sp?) (means High Belts in French) that is slightly taller with shorter straps. These come in sizes 27, 32 and, I believe 37. People still call this a Birkin.
For leather, there are texured and smooth varieties. The most popular leather for Birkins right now is Togo, a scratch resistant textured calf leather. You can also get smooth leathers, and goat leathers, as well as exotics. Some leathers -Togo, Clemence, Natural Barenia, Chevre - (goat,) Box (very smooth and shiny) Chamonix, are always around. Other leathers are produced for awhile, then get discontinued (Veau Graine Lisse, Courcheval for example.) These are usually the pressed leathers with a bit of texture and sheen. Some new ones that I'm not familiar with are Swift and Rodeo. I can't wait to see them.
Then, there is the hardware. You can get 24K plated gold, silver palladium, a more burnished ruthenium and one I LOVE called guilloche. This is palladium hw with a diamond cut pattern on it. Fabulous and rare!!
Then there are colors. More than you can imagine. Reds, greens, oranges, browns, yellow... the most popular Birkin colors are Gold (really a rich tan color,) black and Blue Jean (a sky blue - gorgeous.)
Kelly bags come in sizes 20 (I think, it's a mini) 25, 28, 32, and 35 cm. The most popular sizes are 28 and 32. The Kelly bag is either going to be rigide (outside stitching and more expensive) or souple (softer feel, inside stitching)
Same HW and leather options. Kelly also comes with a shoulder strap!
Whew! If there's something you'd like to know that I didn't eleborate on, just ask. If I've gotten something wrong or mixed up, anyone, please correct me!