Subwoofer Questions for car audio?

I am looking to buy a single 12" subwoofer to put into a vented box and i wanted some suggestions of different brands to look at. I would like positives and negatives of them and i don't want to break the bank on them. I have a Kenwood KAC-9105D amplifier which puts out 900 watts RMS at 2 ohms and 500 watts RMS at 4 ohms. I would also like to know which level of impedance is better and why.