Why limit yourself to those two? There are lots of great subwoofers out there, and many of them are significantly cheaper, better-sounding, or both than the Jello or Alpine offerings.

Four examples:

1) Adire Audio Brahma
www.adireaudio.com
Pros: Sophisticated motor design, exceptional quality.
Cons: Pricey, silly dustcap logo, requires lots of power.

2) Lamdba Acoustics
www.lamdbacoustics.com
Pros: Superior motor design and peerless attention to detail. His Apollos are the best woofers money can buy.
Cons: Nick doesn't suffer fools gladly, so you have to know exactly what you want.

3) Dayton Titanic/DSQ
www.partsexpress.com
Pros: Basically a Lamdba motor, albeit without the Faraday ring that so lowers the inductance of the Lamdba lines. Extremely good value for the money.
Cons: Not as sophisticated as the Lamdba or Adire motors.

4) Peerless XLS
in the US, www.partsexpress.com or www.madisound.com, corporate site www.d-s-t.com.
Pros: Used in US$20k speaker systems, and especially for Europeans probably the cheapest option.
Cons: Again, not as sophisticated motors as the Adire Brahma/Tumult or the Lamdba, so distortion will be higher.

Frankly, I think road-noise would swamp a lot of the motor differences, so my personal choice for a car woofer would be the Dayton, followed by the Peerless.

Jay

PS: Lest you think I used "Jello" as an insult, you're wrong. I've built excellent home and car subs around their woofers, and I called them "Jellos". However, the fact of the matter is they've been passed by.