How many flats do you get?

If you only get about 1 or 2 flats per year a mini pump might be OK. The only real drawback is they take about a gazillion pumps to bring a tire up to operating pressure. If you get frequent flats, that would be frustrating to me.

If you get frequent flats, you'll probably like CO2 better because it's so much faster and more convenient to use. Make sure that your inner tube is properly seated before re-inflating. There's kind of a learning curve to using it so you might never get comfortable using it only once a year.