APA has more members. They also get more people to come to Vegas, around 7000. Due to the way the APA is setup a lot of the players who come to the tournament each year are first timers. As a group the APA members spend less money on their equipment than do BCA members. Individually though some APA members who can't run two balls don't hesitate to drop a grand on a cue.
BCA has better players but only about a 20-25% of the APA's membership. The majority of people who come out to their annual tournament have been many times before. BCA players tend to own higher end equipment and spend more money. Around 5-6000 people attend the BCA's annual tournament.
Until recently the venue was the same for both. It was the Riviera Hotel. I don't really know what the plan is now.