I have Band in a Box 2007 with all styles, Jammer 6 without any extra styles yet, Onyx arranger with all styles.
BiaB 2007: Clunky interface. A lot of things are done with numbers and strange menus. There are, however, lots of styles (if you buy them), though they have a tendency to sound jazzy. Even the rock ones sound like a jazz musician doing his best to play rock. There a regular updates, a forum and more.
Jammer 6: Real Windows interface. I like this. I like the whole approach and the way you can manipulate things yourself, including modifying or creating new styles. Their own styles, however, are limited... Well, in their adds it sounds like a lot of styles, but that is because they split things up. One style in BiaBi is equivalent to about 12 styles in Jammer.
Onyx Arranger: Stay away from this. Nice program with a lot of potential. The first which could convert midi files to styles. But ... they haven't updated for almost 2 years, even obvious bugs, and they don't respond to support questions. My guess is the programmer has disappeared and they bought the stuff and just sell it without it ever being updated again. Sorry, forgot, there was actually a recent update after 2 years, but only minor bug fixes, no development of the program.