If you can find them locally, I'd definitely recommend going that route. As Zaz mentioned, the shipping can be ridiculously overpriced, especially for something heavy like this. The biggest advantage, obviously, is that you can get a feel for which one will be best for you if you can pick one up in person.
I remember when Cynthia first handed me the 10-pounder. It felt SOOO heavy. No I'm able to do 80% of my DVD routine with a 20# one. I personally suspect that 15# might be the ideal weight to use over a sustained time for fat burning, but the 10# will absolutely do that for you, too, if you stick with it.
"Kettlenetics" actually appears to use even smaller, 6-pound kettlebells but I don't think I'd invest in anything that small. If you're as sore as I was the first couple of days after your first workout, you might think that you need something smaller but I really think that, barring specific medical conditions that would prevent you from weight training, most women can handle a 10-pounder.
Bottom line: if you get a 10-pounder and do a series of about 8-10 exercises 2 to 3x/week, you'll be amazed at what will happen.
One last thing: when I was getting dressed this morning, I spied a pair of pants on the bottom rack of my closet that were grouped with my fall clothes. I had forgotten about them and was kind of surprised to see that I'd kept them during my de-cluttering purge in the spring. They'd always been a little tight but I just really liked the style. I slipped them on this morning and they fit great!