I think its a matter of personal choice. I have discovered that the expensive ones (over fifty dollars) that look fancy or are supposed to do more than the less expensive ones, when I am enjoying them, I can't tell the difference between the twenty dollar one and the fifty dollar one.
I know that they have different speeds, for me a three speed works and its a waste to spend more money on something that has five or eight speeds.
Anyhow its alot of fun shopping for a vibrator. Take a look at some of the ones online and get an idea of what interests you before walking into the adult toy store then you'll have an idea of what size, style, etc that you are looking for.
Also alot of fun are the ones you can use in water as well as out of water - makes bath time interesting!