Editing Vehicle Design Guide (section)

=== Armor Grade ===

*'''A''' armor weighs half of what B armor weighs but costs four times as much
*'''B''' armor is the normal armor in terms of weight and cost.
*'''C''' armor weighs twice as much as B armor and costs half as much

It really comes down to a choice between B and C grade as A is much too expensive for anything but racing type events. C armoured cars will be slightly slower and if you're going with a smaller engine too much C armor isn't a very good idea. C armor will also be cheaper to repair and mean the cost of keeping your vehicle in fighting shape will be less - and this can add up quite a lot over time.
The place you'll really appreciate having a lighter and faster car will be when running away from encounters you realize you can't win.

You can't get refunds on your armor so switching the armour type later will be an expensive and time-consuming process. I'd recommend dishing out the extra cash for B armor for your first car and C on your second to see which works out better for you.