Which black dress should I get?

Love both, but like everyone else said, it really depends on the occasion. If I had to pick only one of two, though, I'd go with #1 since I think you could throw a cute colored cardigan over it to make it a little more conservative.
 
It very much depends on the occassion. The first, while I like it, would have the tendency to look a bit trashy if worn to a more conservative funcation so I'd keep in mind the event that you plan to wear it.