My knowledge of the BJD world is pretty limited, Liz, because it is SO expansive and one would need to seriously sit at the computer for a month as Cindy mentioned. Frankly, I couldn't be bothered and just dabble a little.
My advice would be to go to meet up with Sylvia and see her dolls. Ask her questions and handle the dolls.
Here are some questions you can ask yourself:
1) What size do you like?
2) What proportions do you like?
3) What kind of stringing tension do you prefer?
4) What type of jointing system do you prefer - more or less?
Another thing to keep in mind is that clothes and accessories are most available for the SD, SD13 and MSD sizes. This doesn't mean other sizes aren't available, but they are harder to find.