Test-drove it yesterday, then went back and test-drove the Civic immediately after. The Si is still a very good car, but the Mazda is hands-down better. More cargo room (since it's a hatchback), more capable on the road, and has all the features I care about. Consumer Reports also seems to think Mazda produces pretty reliable vehicles (they're still quite Japanese, even if they were bought out by Ford).
The only downsides to the Mazda are its slightly-worse gas mileage and the fact that its suspension is a bit stiffer than I would like. (By contrast, however, the Honda's was a bit looser.) Also, it's FWD, but I can't afford any of the AWD cars I liked and I'm not about to buy a Mustang. :P
So now I just need to buy the sucker. I've found three potentials (two of which are even in the right color, and the other is my second-choice color), just need to figure out financing and insurance and hunt 'em down.