About a year ago I was in the same dilemma. I wasn't sure if I wanted to buy the P5, the Matrix or the Pontiac Vibe.
I test drove the P5 and liked it, but wasn't sure if I could deal with the advertised gas mileage or limited cargo space. Then I test drove the Matrix and didn't really like it, but thought that the good gas mileage and extra cargo space fit my needs more than handling or good looks. So I tried to buy the Matrix, but since the car was so new I was unable to get a good deal. If the dealer had been willing to give it to me for even the sticker price I would have taken it, but that didn't happen. Then I test drove a Vibe, which is basically the same thing as a Matrix. I liked its look better, but the car was just coming out and only a few of them were available at the time. The one I found was so loaded with options that it was out of my price range. So I went back to the P5, got a good deal with Mazda's incentives and bought my car.
Ten months later I am happy I bought the P5? Yes and no. It looks GREAT and handles GREAT. I love it when it's washed and waxed and still look back on it every time I walk away. The cargo space seems to be sufficient for what I'm using it for and though the engine could use a little more power, the overall driving experience leans towards fun. BUT...with that said I am disappointed enough with the gas mileage to say I wish I didn't buy the car. I'm getting on average in the low 20's for the driving I do. If I were getting in the high 20's I would not complain. That's not great, but it's adequate. The resale value has dropped quickly and now I feel stuck with it. In no way am I saying that the P5 is a bad car, because it's not...it's an excellent car for the value. What I am saying is that you should figure out what your greatest needs are and buy a car based on that. Looks only go so far.