surface area or raw power?

jmelton's questions are good for you to answer because you need to narrow your search requirements.  The question as stated is too vague for a definitive group response, although sedate and DYohn have given responses that when combined should lead you to a conclusion.

A very important consideration, when you are contemplating  the use of 6 subs in a vehicle, is how they are placed (let alone the box requirements).  You have to make sure that the sound waves from each one follow the same path so that they all arrive to your ears at the same time.  If you can achieve that, and each sub has the same input, you will have correlated sound.  It's much easier to achieve that with two subs in a car.

Here is a SPL calculator I want you to plug some numbers from some different subs into so that you can get a better idea of what you are trying to find out.  The numbers you will plug into the calculator are:  sensitivity, power per speaker (200 or 600), number of speakers (6 or 2), and check the box "in phase" for calculations that concern correlated sound.  Use 10 feet for all computations to keep things the same.

Here are some random subs you can play the numbers with:

1. low cost 200 watt sub:  http://www.cerwinvegamobile.com/HED12.php

2. another 200 watt sub:  http://www.memphiscaraudio.com/products/details.asp?cat=&id=57&p=2

3. a 600 watt sub:  http://www.crutchfield.com/App/Product/Item/Main.aspx?o=m&showAll=N&g=520&i=136126V2D4&tp=111&tab=features_and_specs

4. and a high cost 600 watt sub , the HX 300D:  http://www.hertzaudiovideo.com/default_en.htm

A note about the sensitivity rating:  it isn't the gospel, but you can use the number in a situation like this to get a general idea of how SPL is achieved with all the different variations.  Try 6 of the CW's, with 91 db sensitivity and "in phase" sound.  Now try them "random phase", as if you were not able to achieve correlation from all six of the speakers.  Keep a tab on the SPL answer for each scenario.

Do the same thing using the 600 watt speakers, but two of them.  You'll start to see a pattern that more SPL is achieved through the use of multiple speakers.  But people who have spent great amounts of time and labor on projects like these know that there is a corresponding greater difficulty in setting up  the multi-speaker array properly.  If done poorly, you just end up with a car full of speakers.