Mono vs. Stereo Car sound system?

I love music. Now that I have a car I want to enhance my car's sound system. I'm not going crazy and replace all my speakers and factory sub. But I am going to add two Kicker 12s" or 10s" subs. I haven't decided yet. The price is so little between the models I figured I should pay the extra 10 bucks for an extra two inches and get the 12s. My Question is whats better mono or stereo amp to power the subs? I like the fact that I can hear differences in the left and right speaker in my car, which means the speakers are stereo. But I can't decided wether I should have a difference in my subs and match the speaker, but all the do is pound extra b. Should the subs pound together as a whole or independently. I am going ot purchase two 12s or 10s Kicker Comp series subs. 300/150 Watts Peak/RMS. I have researched the difference between the 12s and 10s the only difference is the frequencies. With the minute difference being 27/500 (12s) and 30/500(10s). I'm not going to test my subs to my friends subs to see who's is louder, but im not worried about a small 75 dollar disturbing the peace ticket because there to loud for my neighbor. I also need to know what is my max Watts Peak/RMS for my amp(If I have read correctly its 600WP/300RMS) and should I match the the RMS of my subs and amp or have a little extra power in the amp. I have read that you can have about 10-15% extra amp power and not blow your subs. I am new to this car audio and trying to get some info before I buy so correct me if I am wrong without the brutall comments I find on Yahoo Answers. Thanks, open to all suggestions.