A Pacifica is more of a "crossover" vehicle - almost like a large AWD station wagon. A Trailblazer is an actual SUV. I've driven both a crossover (Ford Freestyle, now called the Taurus X) and an Explorer, and believe it or not, I found the crossover to have more room. If carseats are an issue, you may have an easier time in the Pacifica since those vehicles tend to be laid out a little more "family friendly".
I also recommend vehix.com to compare the cargo room - you may be surprised.