Most systems use the radial electrolytic caps.
The voltage rating needs to exceed the max votage that it is going to see and the capacitance needs to be large enough for any spiked/ demands.
Westie has recommended a 16V rated one with a capacitance of 4300uF
I have also seen 25V rated 2200uF capacitors recommended.
16V+ will be fine.
Ripple current capability will also be important. I have seen blown capacitors on PWM systems before due to an inadequate ripple current rating of the capacitor. This was none RC car related though I hasten to add.
This is what Hobbywing recommend as their 'super' capacitors
http://www.hobbywing.com/english/Art...?ArticleID=417
Basically 16V 1320uF with high ripple capability.
Hope this information helps some what......