I've shot IDPA for almost four years and USPSA (US region of IPSC) for a little over one year. As far as which entity is growing faster, most people might say IDPA. However, with the advent of USPSA Production class (almost identical to IDPA SSP - stock guns limited to 10 rounds), USPSA is increasing its membership at a decent clip.
As far as cost goes, don't let anyone tell you that you can't afford to shoot USPSA. Once again, the introduction of Production class a few years ago almost neutralizes equipment advantages. I shoot the same gear in both sports, but carry two more mag pouches in USPSA because the stages usually require more rounds.
Neither of these SPORTS are real life. Neither of them simulate combat. In IDPA, we have what I call Hillbilly Ninjas who think IDPA is all about training to kill people on the street or in their home. While Hillbilly Ninjas probably only constitute 1 in 25-50 IDPA shooters, in my opinion, they sour the whole bunch. In real life, I will not be doing tactical reloads, I will not wait until I am behind cover to do a reload, I will not go on house/building clearing exercises if I have an alternative exit option and most importantly, I will likely turn and run away from, instead of toward, the enemy, if at all possible. If you want training, go to one of the "tactical" schools in Arizona or join the Army.
Now, I won't tell you which sport to choose because you need to try them both. Shoot a few matches of each sport and go with what feels most comfortable. I've won state titles in IDPA and USPSA and enjoy them both. However, I am beginning to enjoy USPSA a little better than IDPA. To each his own.
Don't let someone else's judgment skew your choice. Go shoot and decide for yourself.